Stockmarket question. Anyone know anything about market maker clearance execution?

I’m writing a memo, and in it I have to explain a niche of the securities market know as “market maker clearance execution”, AKA “flip execution” or “the flip business.” And I’m pretty clueless.

As I understand it, the flip business is where a broker-dealer allows a firm who deals on one exchange access to the broker-dealer’s trading system, so as to allow the firm to clear and settle their deals on a second exchange.

But I’ve searched the SEC, NYSE, and NASD websites and turned up nothing at all, much less a cogent explanation.
Anyone know what I’m talking about? Anyone able to explain it to me in terms I can understand, or have a link to a website that explains it?

Thanks in advance,