What (event? something else?) changes the User "Modified_at" parameter?

If you do an API call for the List Enterprise users, as part of the data you get back is a field/parameter called "Modifed_at" which shows a date/time stamp.  I see from an earlier post that "anything that modifies the User Object" is the answer, but can you tell me what kind of Events might modify the User Object?  What I'm actually looking for is something that is close to a "Last Event".  If I run an Admin Console Report for User Activity (all events) for a single user, the last Event date/time I see in the list seems to correspond to a Login Event for that user, but I'm not sure if anything else would trigger a change to that "Modified-at" date.

Can we get some clarification on what Category of Events or Event Types would change the "Modified_at" field for the User Object?