PN = Pairing awarded that was not in any layer. On the bid award you'll see P1, P2 etc. Those are pairing awarded that you bid in layer 1 or layer 2. PN means PBS went all the way through your 7 layers and couldn't complete your line so it had to award you something that you didn't bid for. A PN means you screwed up your bid. PBS can't 'screw you' into a PN. You messed it up yourself by making your bid too restrictive for your seniority.

CN = Coverage date pairing awarded that you didn't bid in any layer. If PBS has 20 pairings to award that start on May 1st, there are 40 lineholders in your domicile, and 20 people senior to you bid for May 1st off it will force you to work on May 1st as a coverage date because it needs the remaining 20 lineholders to cover the 20 pairings. You can potentially bid correctly and still get one of these because coverage dates aren't known before you bid. If you want to reduce CNs then make sure your layer 7 is broad enough to allow just about every pairing. It will then in theory award you something you actually bid for that starts on May 1st instead of going to a CN. Once PBS goes outside of layer 7 it can tack whatever it wants onto your line.

Even bidding 75% in domicile I've never had a PN and I've only had a CN one time for a standup because I didn't allow standups in layer 7.