There's an undocumented (or poorly documented) feature that participants in reviews can mark revisions of files as "approved". This effectively bookmarks that version of the file for that specific participant, and also indicates all other participants that the file has been reviewed by the individual. This plays into the per-user setting of the default comparison view of "Last Accepted vs Latest".
The feature is:
1. Poorly named. Reviewed does not mean accepted, it means it's been looked at . Subsequent changes may still be necessary, but I might not want to have to review everything from scratch again just to see if the one defect/bug was fixed.
2. Not obvious: Clicking an un-named checkmark button in the file view, but only on the overall discussion of the file, shouldn't trigger completely different behavior from the same button on any other part of the file.
3. Not documented: Only because I clicked it accidentally did I even discover it showed up on the Overall materials section. Through playing around with it did I discover what it did as a bookmark on that version of the file. Since it's not self-documenting (see #2), it should either be fixed to be self-documenting or much better documented in the user guide. I would consider this a major killer feature, and it's almost completely hidden.
Suggested fix would be separate it from the File Overall discussion into a separate button. Something called "Mark Version as Reviewed" or something similar. Then use a different icon so it's not confused with the non-functional "Accepted" checkmark button that currently also appears on every comment/bug.