This is a special merging mode that counts on newer versions of MAME not respecting the romof tags, leaving parents with files that are not actually required by the children. This new merging mode ignores those romof and cloneof tags, using devices only to add more required roms.
Previously, sets that had multiple items would trigger issues because of shared information in the items, mostly the Machine parts. By making all of it a clone, it takes a little more memory but ends up resolving all issues in desc-to-name mapping and replacement.
