Please excuse my ignorance in advance.

New FF here and recently I had to change a booking and the agent told me the price. She said this was a "P" fair and if there were open seats I would be in F. She reserved a F seat for me.

Can someone please explain how a P fair works?

Was the reason I got the P fair because there were no lower fair seats available? The trip is MSP-SLC on a Friday evening so maybe it was full?

Thank you