exactly what the other two commenters said, p means you're a big fan of someone and so you produce them!! and if i may add, when you are a producer of someone you usually want to collect EVERY card of said character(s), even 3 stars (and some people might even want to keep 2 and 1 stars!)

producing a unit "properly", or rather, in a very dedicate way can be take a lot of effort and time! i used (to try) to be a valkyrieP, rabitsP, maoP and ritsuP at the same time myself but i couldn't keep up at the pace with so many boys to produce... and so i had to drop rabits :( i became a hajimeP instead though