The prep's are on the MPSCS, same as the mobiles. The troopers check in with regional dispatch at the start of their shift, usually using their mobile radio. When a user transmits on the MPSCS, the RID (Radio ID - like an ESN) shows up on the display of all radios on that talkgroup. The dispatcher records that number, and also the RID of the trooper's portable, so that if they set off their emergency alert on either radio, the dispatcher will know who it is, in case the trooper is unable to speak. It's not necessary to talk in order for the RID to display, so they just key up the portable for a second. Our officers usually check in on their mobiles and give us their portable's RID verbally.
Back when MSP was on VHF low band, their portables were on VHF high, and each car had a mobile extender in it to bridge the portable to the mobile. They referred to those as the "Portable Repeater", which may be the origin of the term "PREP". That's one theory, anyway.