Both distribute power downstream of the UPS — but a Remote Power Panel is a room-level circuit breaker panel, and a Power Distribution Unit is the rack-level device that actually terminates in outlets a server plugs into.
Power leaving a UPS still has to travel through several stages of distribution before it reaches a GPU server's power supply, and two of the most commonly confused stages are the RPP (Remote Power Panel) and the PDU (Power Distribution Unit). An RPP is essentially a circuit breaker panel positioned out on the data hall floor, closer to the load than the central switchgear, splitting a large UPS-fed feeder into many smaller branch circuits — each one typically feeding a single rack or a small group of racks. A PDU is the device actually mounted in or beside that rack, taking one of those branch circuits and breaking it down further into the individual outlets that server power cords plug into, often with local monitoring, remote outlet switching, and metering per outlet or per circuit. RPPs live in the room; PDUs live in the rack. Confusing the two — especially when sizing circuits or troubleshooting a tripped breaker — sends an engineer looking in the wrong physical location entirely.