I am reluctant to join this thread given that my name doesn't end in "ook" but XLN do two types of add-ons, kits (AD Paks) and sets of midi sequences (midi Paks)
The ADPacks have samples of a kit and usually a small collection of midi sequences (grooves) which work nicely with the kits pieces
Midi Packs are just sequences (grooves) which can be used with any kit but work better with some than others.
For the record you can also get individual kit pieces.
AD has a slightly different mapping ( note to kit piece mappings) to other drum sample/sequencers hence why their in-house midi paks work out of the box