P1 is a term for preference level.*P1 (old school radio term) traditionally was the group ( audience ) that you tested music on and your sales people pitched to advertisers.
If I listen to Station A more than any other station, then I am a "P1" for Station A. Doesn't matter how much I listen to them, just that I listen to them more than any others.
If someone else listens to Station A twice as much as me, but they listen five minutes more to Station B, then they are P1 to Station B, and P2 to Station A, even though they give twice as much listening to Station A than me.
Testing will usually look beyond just "P1s" and set minimum listening thresholds to ensure that low-level listeners are not included.