PNR = Passenger Name Record

As beckoa said, a single PNR can be constructed with more than one passenger name. All of the passengers in a single PNR must have the exact same itinerary. If one of the passengers needs to make a change while the other one doesn't, the the PNR has to be split into two PNRs, one for the person making the change and one for the person not changing, since you can't have two different itineraries within a single PNR.