Discount realtor vs cash back realtor: what is the difference?

"Discount realtor" and "cash back realtor" get used as if they mean the same thing. They do not.
Discount realtor
Usually means a lower listing fee for sellers. Sometimes the service is the same; sometimes it is reduced, such as fewer showings, a call centre instead of one agent, or no open houses.
Cash back realtor
Usually means a buyer's agent who gives you part of their commission after closing. Service is normally full, because the agent still earns most of a normal fee.
Why buyers need the second kind
Buyers rarely pay their agent directly; the seller usually does. So a "discount" on a buyer's agent saves the buyer nothing unless it comes back to them as a rebate.
Why sellers want the first kind, done right
Sellers pay the commission, so a lower fee is a direct saving. The question is whether the service is still full. Ask exactly what is included.
RebateSpace does both
Buyers get a share of the commission back, and sellers pay a lower listing fee, with the same partner agent and the same full service.