Action Details
Requirements and how-to articles for Steam's action:

Requirements
You need this to get it done:

Game Name
The name of your product/application. Include trademarks and/or registration symbols.
Launch Discount (optional)
If you wish, you can set a discount to run for the first week your product/application is available for purchase. Steam generally recommends a discount between 10% and 15% if you decide to have one.
Developer & Publisher
The developer and publisher of your product/application. If you do not have a publisher, you can put the developer here as well.
Website URL(s)
You can provide links to your product's website, forums, stats and online manual. (All URLs must start with http://) You can also include the name of your product's Metacritic entry.
Keywords
You can enter relevant keywords which customers may use when trying to search for your product. Keywords should be in a list separated by commas (e.g. video game, videogame, gaming.)
Supported Platforms
From Windows, Mac and Linux, choose which platform(s) your application supports.
"Coming Soon" Options
If your game is coming soon, you can display the release date on the store page. You can also use a custom string instead of the actual release date (e.g. Summer 2020.)
Supported Languages
Select the languages your application supports (separate tickboxes for interface, audio and subtitles.)
Genres
Choose the primary genre of your application (which is used in the display of the navigation breadcrumbs, just above your product title at the top of your store page) as well as any secondary genre(s).
Players
Select whether your game is single-player, multi-player, whether it has co-op, etc.
Supported Features
Indicate which features are supported by your application (e.g. Steam achievements, VR support, captions, etc.)
Controller Support
Indicate whether your application supports controller input, and if so, what type(s) of controller and how much support is available.
Third Party DRM
If your application uses third part DRM, you can list the provider and machine activation limit.
Legal Lines
e.g. "Game Company © 2019" (this appears at the bottom of the page)
Support Contact Info
You must provide at least one way for customers to contact your support department if they have an issue with your application (support website URL, email address or phone number.)
"About This App"
Describe your product/application. Valve recommends starting with a brief overview, and then going into more detail, with a list of key features and then whatever else you feel is relevant. BBCode formatting available. This can be localised.
Short Description
This is shown in various places within the Steam store, and also for web widgets. BBCode formatting available. This can be localised.
Length: 200 - 300 characters
Review(s)
You can add reviews of your product/application to the store page. You should provide the name of the organisation that wrote the review, a link to the review, the score they gave your product/application and a quote from their review. This can be localised.
Awards
You can add images to represent award(s) which your product/application has received.
Recommended dimensions: 125 x 125 pixels
Special Announcement Section
This is for special announcements or features that you want to call out above your product description. These are typically used to show information about special editions, pre-purchase offers, or to present a banner for Workshop integration. You can either provide your own section header, or select a predefined string which is already localised (you can also country-restrict this section if you wish for it only to be presented to users in specific countries.)
Banner (optional)
You can add a custom banner image to your store page.
Recommended dimensions: 600 x 150 pixels
Age Rating
If your product/application has age ratings from rating agencies/organisations, you can add them to your store page (rating, descriptors and age gate.)
Header Capsule
This image will appear at the top of your page in the store, and should focus on the branding of your product. For best results, please use the same artwork that you use for any retail box or marketing and make sure the logo is clearly legible.
File format: .png
Dimensions: 460 x 215 pixels (a 292 x 136 version will be automatically generated from this)
Small Capsule
This image is used for lists throughout Steam such as top sellers, new releases, etc. Due to this image's small size, for best results focus on making the logo clearly legible.
File format: .png
Dimensions: 231 x 87 pixels (120 x 45 and 184 x 69 versions will be automatically generated from this)
Large Capsule
This image appears at the top of genre and publisher pages and occasionally on special promotional pages. This should be graphically-centric and give the user some sense of the experience, ideally using the same key art and logo which is being used for any retail boxes or marketing.
File format: .png
Dimensions: 467 x 181 pixels
Main Capsule
This image appears at the top of the Steam store home page in the Main Capsule carousel. It should be graphically-centric and give the user some sense of the gameplay, ideally using the same key art and logo which is being used for any retail boxes or marketing. Avoid placing the logo/title in the lower right corner of the capsule image where it might be obscured by promotional overlays.
File format: .png
Dimensions: 616 x 353 pixels
Community Icon
This small icon is used in the upper-left corner the store page and community hub pages for your application. This icon will not appear properly on your store page until your app is published as 'Coming Soon' or as fully released. (Note: The Community Icon can be automatically generated from the Client Icon .ico file.)
File format: .jpg
Dimensions: 32 x 32 pixels
Client Icon
This icon is used when creating desktop shortcuts to launch your application. This icon can be used to supply the community icon and client image if desired.
File format: .ico
Dimensions: 32 x 32 pixels
(You can also submit an ICO file with multiple sizes to accommodate scaling on different resolutions.)
Community Capsule
These capsules are used in the Steam Community when showing recent playtime on a user's profile and when listing out apps on their wishlist. Typically, this is the same small capsule that you upload to your store page. This capsule asset is also displayed in the Steam Client Library List View.
File format: .jpg
Dimensions: 184 x 69 pixels
Page Background
This is the image that may be used in the background of your store page. This should be ambient so as not to compete with the content on the page. If you don't upload a background image, Steam will automatically generate one from the last screenshot.
Dimensions: 1438 x 810 pixels
Bundle Package Header
This image appears at the top of the bundle detail page on Steam. This image should focus on the branding of your product. For best results, please use the same artwork that you use for any retail box or marketing and make sure the logo is clearly legible.
Dimensions: 707 x 232 pixels
Screenshots
Screenshots are displayed on your store page, and on other pages where your game may be featured such as the Steam homepage. You must provide at least 5 screenshots of your product. For best results, please provide screenshots in high-res, widescreen format.
File format: .jpg
Dimensions: 1280 x 720 pixels or 1920 x 1080 pixels
Trailer(s)
As part of the release process on Steam, you will be required to upload a trailer for your product. Trailers are displayed at the top of your product's store page and are often one of the first things your potential customers will be seeing. You will want to think about how long it takes for the customer to learn about your product, and if they can learn about it without audio.
Resolution: As high as possible, up to 1920 x 1080 pixels.
FPS (frames per second): 30/29.97 or 60/59.94
Bit rate: 5000+ Kbps
File format: .mov or .wmv
Aspect ratio: Recommended 16:9 but 4:3 is also acceptable.
Audio: Stereo, .aac, 44Khz or 48Khz sample rate.
The "Base Video" field only needs to be set if you have multiple languages of the same video. Any language in addition to English should have the Base Video field set to the English video.
The "Countries" field only needs to be set if you wish to restrict visibility of your trailer to only certain countries (e.g. if you have videos which are PEGI/ESRB rated.)
A thumbnail image will automatically be generated, but you can use a custom one instead if you wish.
DLC (Downloadable Content)
If you have downloadable content for your product/application, you should add it to the store page. You must provide either the DLC title or appID.
Pricing
You can choose the price you wish to sell your product/application at in USD, and then Steam will suggest prices for all of its other available currencies, which you can then either leave as they are or edit them. Once you have decided on your pricing and saved it, you must wait for it to be approved by Valve, which typically takes 24 - 48 hours.
Demo
If you have a demo for your product/application, you should add it to the store page. You must provide either the DLC title or appID.
Release Date
Set the date you wish to release your application on Steam. Make sure it is a week day (not Saturday or Sunday) as nobody is in the office at Valve on weekends to monitor releases. All times are in US Pacific (PDT.)
System Requirements
For each supported platform, you must include the minimum system requirements. If you wish, you can also include recommended requirements for best performance. (OS Version, Processor, Memory, Graphics, Network, DirectX Version, Disc Space, Sound Card and any additional notes.)
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

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Steamworks Documentation - Editing a Store Page
Information on how to prepare your store page before release and update your store page after release.
Official
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Steamworks Documentation - Graphical Assets
Specs and requirements for the graphical assets for your product's store page.
Official
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Steamworks Documentation - Trailers
Specs and requirements for trailer videos for your product's store page.
Official
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Steamworks Documentation - Pricing
Information regarding setting the price for your product on Steam.
Official
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