720p60 records the highest frame rate possible with a regular video camera. 720p60 is easily downconverted to standard def 480i. A Kona card does a nice job on that.
720p30 is recorded to the P2 card as 720p60 with flags embedded in the recording to tell the capture software to remove the duplicate frames on ingest. So, you are recording 60 frames per second but only using half of them in your edit.
720pN30 records only 30 frames per second (29.97 really) to the card. So, the pN30 mode is able to record twice as much time on a P2 card.
Similarly, you can record 720p24, which records 60 frames per second to the card, or 720pN24, which records only 24.
Barry Green’s The HVX Book is good. Also this on formats:
https://manuals.info.apple.com/en/FCP6_HD_and_Broadcast_Formats.pdf