ClipMate can automatically delete data from a collection in 3 different ways:

Length: when the collection grows past its retention limit, say 200 items, old items are deleted to bring it back to the retention limit.
Age: specify a number of days to keep clips.
Never:  never automatically delete clips from this collection.

The "purging" takes place at shut-down, and will move them either into the Overflow collection ,or the trashcan.