my understanding of execution is only is say for a company that has a system that tells you what to trade, i.e system says buys 5000 lots Mini S&P at 1176 and then you have to buy it at that price (using whatever method to hide your trades, if you need to).
anyway, back to what u asked, say the market is 19/20 and the client says "pay 20 for 5" then sure you can hold that on your "book" if you think the market is going down, but if the market moves up then you will take loss if u can't pay 20 yourself for that sheaaahh.....BUT this is market making instead of execution only!!!!!!!!!!!!!!!! bear that in mind!!!!!!!!
So, client pays 20 for 5, then you are short 5 at 20, you can hold it for a while or just go on the bid at 19 and try and steal a tick.....of cource the market could move and u might have a positon that you dont want, which again boils down to trading, which is not what an execution dealer does.....an execution only dealer will simply do what he is told (depending on market conditons)...I think it boils down to the specifics of the job and company.....
does this help?