Indeed these are all prefixes for various of bug types (P is for Product by the way). It is depend where these bugs were found
and/or how it was release to the field.
The reason that we expose these values is, as mentioned previously in the thread - to give the option to compare between several SKs ( example - to see if specific issue was solved in the Jumbo)
PRHF - Private hotfix , found on the field and fixed as private hotfix ( PRHF)
PRJ- Fixed in the Jumbo
PMTR - Found and fixed on main-train
MBS - found/added as part of other projects ( MBS for example , but can be other projects as well)