I'm not really into shows so I'm no expert at this and I know this is a bit late, but better late than never -
PLAKAT = short fin. What ever the fins may look like, plakat means short fin. Perhaps to avoid missunderstanding the IBC clarifies name by calling them "Short fin"
The original form of short fins is round tail.
HM = a perfect half circle caudal ("D" shape) - thus caudal edges must be pointy, not round.
The plakat or short fins were cross bred to long finned HM (HM were initially created long fin). Now the IBC classifies them into:
Traditional (second picture posted by MO), Asymmetrical (third picture), and Symmetrical. ALL plakats/short fin must have 180 spread, traditional with 2 rays round edged caudal, Asymmetrical (longer anal lenghth) with 4 or more rays and pointy caudal edges, symmetrical with 4 or more rays BUT this class must have even fin lengths and longer dorsal.
The term HM-PK is what people use to indicate that they have 180 spread. It isn't. to my knowledge, a term used by the IBC that calls them short fin. The IBC considers the symmetrical and the asymmetrical to be HM short fin (they must have a "D" shape caudal). But public just call any PK with 180 spread as HMPK.
So basically it is only different terms used by IBC and public.