In TimeMachine, what happens during the "clean up" phase? When TimeMachine runs a backup it goes through several phases.  There is the "prepare for backup", "backup" and "clean up" phases.  There might be more.

What happens during the "clean up" phase?
When Time Machine goes through the "clean up phase" it's doing "house cleaning" which will consist of activities like:


delete old/expired backups (i.e. past X number of days)
removes old(er) backups if there is insufficient space
trimming incomplete backups