Ok, i'll try and explain it in super simple terms.
Split: Every game is "split" up, there is one "parent" rom of a game which is the game but for 1 region and all the other regional roms are in their own "clone" which is much smaller. The "clone" rom requires the "parent" rom to run.
Merged: Every game has its "parent" and regional "clones" in a single zip file. You still require bios files though for games that require them. EG: NeoGeo and Capcom CPS2
Non Merged: Every game is in its own self contained zip file, it contains everything required for the game to work. The regional "clone" roms have the "parent" rom included as well as the bios files required.