Looking at my games in List View, I noticed that it shows a question mark over some of them with whether or not they support the Steam Cloud.
What does this mean?
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I believe the ? means that Steam isn't sure if it's synced with the Steam Cloud. When you launch the game, or if you toggle cloud support for a game, the icon turns from the "question mark" to just a cloud icon. I tested this with a couple of games in my library, and this seemed to be a pattern. I can't find any Steam FAQs on this topic, so this is more of an educated guess backed up by some research.
I presume that on launch, Steam checks in with the cloud servers to see if there's any updated information to sync, and then it "knows" that the data is properly synchronized. At other times, it's possible that it has changed since the last time Steam checked, so it shows the question mark.