Ethic: soft doller: client brokerage

"Allocation of client brokerage should not be based on the amount of client referrals the manager receives from a broker. " can somebody give an example illustrating the direction of cash flow? is the investment manager will pay out the brokerage for the client? what is “client referrals” here refer to? Confused… what is "client brokerage from agency trades " and “client brokerage from principal trades” ? example? Thanks.