Steam App ID
All games using the Steam Online Subsystem must have a valid application ID because the Steamworks API won't initialize if it doesn't know your application's Steam App ID.
Before initializing Steam, UE4 will generate
steam_appid.txt (during a graceful shutdown of the engine, UE4 deletes this file).
It's important to note that
steam_appid.txt must be located in the same directory as your application's executable file because Steam will look for the text file in the current working directory.
Additionally, the file should not be included in any Steam images.
If you open
steam_appid.txt, you'll see a SteamDevAppId entry, which is a field that hints your application's ID to Steam.
This makes it unnecessary to launch the game using the Steam client (although it must be running).
If you want to test your application, you can use a SteamDevAppId of
480, which is a test App ID that is shared by all developers.
Although you can test most Steam Interfaces with the aforementioned test App ID, your application will need a Steam App ID before being shipped.