Hi shahrizai,
I know we discussed this elsewhere, but someone pointed out this post to me, so I figured I’d update it in case anyone else has same question.
steam_appid.txt file is just a text file with steam appid assigned from Steam in it. It’s required to be in game root folder when running with Steam. In non-shipping builds, it’s generated automaticallly and deleted when engine shuts down gracefully, but it needs to be manually created for Shipping builds. engine uses SteamDevAppId value when it generates file.
steam_appid.txt file is only required for Shipping builds that are not run through Steam, so it’s really only a development thing. This isn’t intended to be released with game; after properly uploading and instrumenting your build through Valve AND launching from Steam client, text file is no longer necessary. Valve SDK site might have more information on why it’s necessary for development. Oh, and it is required when distributing dedicated server builds.
That said, it’s harmless to distribute it, especially now while you’re using 480 testing appID.
Additionally with Shipping builds currently, you’ll sometimes need to run it through Steam client to get Steam Overlay to show up. You can do this by adding a non-steam game to your library and opening it from there.
Hope that helps!