Here you don't talk about simple analysis of non marketable client orders the banks have on its brokerage book and front running , right? You are talking about deeper analysis of the client needs in order to offer them what they want ( with an edge for the bank...). I guess for most people on this site, bank customer flow trading is simply for example, when an IB trader sees a large HF client posting a limit order on XYZ stock and the trader gets a bit of it before the HF...
No,it's usually a bit more complicated than that.
Take a desk trading corporate bonds. They'd be paid for bookrunning (essentially taking pricing risk away from the issuer). They'd be paid for market making (essentially taking liquidity risk out of the market, for a price). They'd be able to make some money creating demand by knowing what customers were holding which bonds.
And a lot of the time what you're buying and selling to the customer is different from what you're trading in the market. i.e. in my case I was trading exotic interest rate options.
Typically we'd offer a structured product to an institution (corporate or public sector). This would have an embedded derivative within it, which often you couldn't perfectly hedge (eg you'd often have to do a swap plus a european straddle, when the optimal hedge would be selling a bermudan call). So you'd make a trading profit on the mark to market difference between those; essentially charging a premium for taking on the hedging risk, and for access to the interbank market where the hedging can be done.
At the other extreme you might do a pure back to back deal with a hedge fund, which you hedged completely in the interbank market. Essentially the HF is renting your credit rating, and your access to the interbank market. At the time (early 2000's) we didn't use proper discount curves so the credit spread within the deal would look like profit.
So in an IB sometimes you're being paid for information, sometimes for having acces (oligopoly?) , sometimes for having a good credit rating, sometimes for taking certain kinds of risk... 'positioning' isn't really a big part of it except in very vanilla stuff like spot FX.
GAT