Okay I'll try to explain it a little better every month MAME roms come out they come in 3 flavors either Split, Merged or NonMerged then these come in two flavors either updated or full sets then of these there are 5 types (with the caveat that the chd sets only come merged) they are rom chd software list roms software list chd's and bios set (bios set is redundant if you have the full sets though they are more for separate MESS usage). Hope that convoluted explanation helps