Payment for order flow is the practice of a market maker (a large company that provides liquidity to the market) paying a brokerage to route customer orders to them. Some exchanges will do this in lieu of charging transaction fees to their customers - but some still charge those fees anyway. This practice (PFOF) is illegal in many countries, as it encourages brokerages to send customer orders to the highest bidder. It also allows brokerages to harvest data on retail investors, and potentially bet against them en-masse. Many companies accepting PFOF have been fined for their practices, but as the fines are far less than profits, there is little incentive not to behave unethically regarding PFOF.