MAME Split Merged and Non-Merged Sets

Some MAME Torrents are released in up to three different flavors:

Note: No setting is needed to tell MAME which set type is being used. MAME always looks for files in all expected possible places.

Comparison Tables
For illustrative purposes, below is a comparison table for the "MAME ROMs" torrents:

A comparison table for the "MAME Software List ROMs" torrents follows:

Conclusion
In short, the three set types have the same contents, but organized differently. In fact, if you have one of these torrents, the other one(s) can be created using a ROM Manager such as clrmamepro or RomVault because the contents are the same. That's why downloading more than one of them is frowned upon, but seeding is OK.

Note that Non-Merged is about two times larger than Split or Merged, which makes it a bad choice if the intention is to download the entire torrent (any of the smaller torrents can be downloaded to create the Non-Merged one).