Market Maker IDs: C & P

I'm starting at Level II (using powerview) and I see Market Makers with the ids of "C" and "P" .... besides them I see INCA/SIZE/BTRD/GSCO etc....

who are C and P?

 

Charlie and Pete....they are from accounting.

 

http://www.nasdaqtrader.com/trader/symboldirectory/symbol.stm

MP Type Legend
A - Agency Quote
C - ECN
E - Exchange
M - Market Maker
N - Non-Member
O - Order Entry
P - NASDAQ Participant
Q - Query Only
S - Specialist

 

What's a "NASDAQ Participant"? Why P instead of GSCO or JEFF or UBSW or something?