prime: prime broker. in my limited experience the main primes (UBS, RBS, Deutsche etc etc) are ill-equiped to deal with allocations. you may have to approach an introducing prime broker aka second-tier prime broker etc... they are usually equiped to deal with both institutional & retail size, by necessity...
basically the prime extends leverage and allows you to trade according to size of 'equity' on master acct, and applies the allocation weightings as pre-defined by you (pre-trade...) and updated from time to time (client deposits / withdrawals), so as to allocate trades & P&L to all your sub-accts...
not sure how well this wld apply to the US... but then again, where there is a will...