LAYOUT_CHANGE_HISTORY and LAYOUT_REVISION_HISTORY are arrays, that have all information about changes or revisions in given layout. Because of that it is possible to extract all change and revision ID's and process this information.
The difference between changes and revision, that usually layout can have only unique revision numbers, that are not duplicating (i.e. R01, R02, R03, R04), when changes - several changes per revision.