From Team Fortress Wiki
- appid (uint32)
- AppID of game
- version (uint32)
- The installed version of the game
- Boolean indicating if request was successful.
- Boolean indicating if the given version number is the most current version.
- Boolean indicating if the given version can be listed in public changelogs. 
- required_version (Optional)
- Integer of the most current version of the app available.
- message (Optional)
- A string giving the status message if applicable.
- Possibly, research on the meaning of this field is still pending