Of all the data in a game, PT4 and PT6 (walkabouts and battle animations) generally grow the fastest. Hence, instead of storing them all lumped together, they are stored as individual records. This is the only difference between these lumps and the normal format.
So, NAME.PT6_14 would contain the 15th attack animation for the game "NAME".
Disclaimer: Eventually, we found that loading every PT record individually made more sense. Why scan through a lump-of-lumps every time you needed one (or worse, multiple!) records. So, eventually, all PT lumps will be split. This, unfortunately, makes the folder a bit messy....