As far as I know, it is 回扣 from some real estate agents' commission when they represent you as a buyer agent to purcahse a home from the builder's (seller's) agent. As a result, this commision 回扣 'may' also be available if you purchase a new home from other builders. (To be confirmed by your real estate agent).
If you don't have an agent when you buy a new home from the builder, the builder's (seller's) agent receive full commission because there is no split between buyer's agent and seller's agent.