Very good question!
This is a fairly new feature. I believe its intent is to inform you of expectations IF you were to use the app in "offline" mode. Not all files CAN be downloaded to the device and therefore would not be available to users for viewing if connectivity were lost. The most notable of these would be any that are accessed through a URL.
It would be nice if we could see which files are not available but that doesn't seem to be available to us yet.
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