the short answer
RPE (rate of perceived exertion) rates how hard a set feels on a 1-10 scale, where RPE 10 is a maximal effort with nothing left. RIR (reps in reserve) counts how many more reps you could have done. They describe the same thing from two angles: RPE 8 equals roughly 2 reps in reserve. Use RPE to autoregulate intensity across many lifts and use RIR when a client finds "how many reps were left?" easier to judge than rating effort.
This article is general training guidance for coaches, not medical advice. Effort-based training raises injury risk if pushed without context - cap intensity sensibly and refer pain or medical questions to a qualified clinician.