An asset issuer defines the market_fee_reward_percent - what percentage of the market fee he wants to share with the registrar in asset options. Registrar defines reward_percent for the referrer for each user.
Market fee reward is accumulated on the user account. The user decides when they want to claim the market fee reward and move it to their active balance. Each user pays network fee to call this operation. More details about market fee sharing BSIP-0043 you will find here https://github.com/bitshares/bsips/issues/kissanime