RPE (ratings of perceived exertion) and RIR (reps in reserve) can be useful training tools for some sports where the goal is to train to failure or near failure, but there is no catch-all number that works for all exercises. Generally, RPE is used for managing heavy efforts of weight or reps, and not for exercises where training to failure or near failure would inhibit the continuation of training like working on endurance for running a marathon. Instead of static numbers, RPE/RIR can function as a percentage of total effort to be applied to a wider range of workouts.