It stands for either "player base" or "played by," depending on who you ask. Basically, a character/actor/actress/whatever that is used so you can have nice icons for your original character's role play journal.

There are a few things I'd give Haruhi up for (and by a few things, I mean Ayumu), but the only reason why Alien's a current deck and SOS isn't is that I ended up with more Yuki cards. XD