When make a mirror in a subassembly, new parts are created that are 'proper' mirrors with prefixes, eg MirrorPart01.sldprt. The file mgt problem I create for myself is I have large assemblies with MANY unique mirrors in them, get the bright idea to do a new design that the main assembly would be a great base for (eg a machine with 50% capacity of the original will still use a lot of the original parts,