RPE (rating of perceived exertion) and RIR (reps in reserve) are two ways a training plan can instruct how difficult a certain exercise should be. Both are self-estimations of proximity to failure within a certain rep and load combination. For example, if a program lists 3 sets of 5 reps at a 9 RPE you should choose a load that you can at least do 6 reps with. This way of instructing loads for an exercise can be incredibly valuable because many exercises do not work with traditional methods such as percentage of 1RM. I suppose if you were to test (max out) every exercise in a training plan to find your 1RM you could then theoretically use percentages but no one has time for that and even if you did you wouldn’t garner an adaptive stimulus from most exercises. Let’s face it, you aren’t going to gain much muscle or strength by performing a 1RM on your DB curls, machine rows, and other accessory movements.