People you are friends with on Steam can usually see achievements, activity on a game, or whether you own the same game as them within the online store. This new feature would mean that you could choose to hide specific games from others, rather than your entire library.
Considering there are plenty of mature, adult, or perhaps simply slightly embarrassing games that gamers might want to hide from their network, it will now be much easier to be selective about sharing your library.
The new feature is almost the opposite of 'hiding' games in your library, which makes them invisible to you but visible to everyone else. This does the reverse, meaning you can still see private games, but others can't.
How does 'Mark Private' work?
Marking a game as private will make it look as though you don't own the game at all. It will no longer appear on your Steam Community profile, in recently played games, or your status on the Friends Lists of others. Achievements from within that game will also not be visible to others.
You can try it out for yourself by heading to the 'Manage' submenu on the game's listing in your library. Bring this sub-menu up by right-clicking on a game or hitting the gear icon on the game details page. Alternatively, you can also do it under the 'Privacy' settings in the game properties section.
You can also mark a game as private before you even buy it, to avoid the risk of anyone seeing it even briefly. There's now a 'Mark Private' option on the cart page when buying new games.
It's worth noting that, for multiplayer games, if you invite a friend to join a server or game with you, that friend will be able to see your in-game status until you exit the game or disconnect from Steam.
