RPE is a scale that ranges from 0 - 10. 0 is the lowest effort, and 10 is maximal effort. At the end of a training session, it is up to the athletes to determine where they are on the scale based on how much effort they felt they had to put into the entire workout (including warm-up, main sets, and cool down).
The RPE scale takes into account other stressors in an athlete's life, which can impact their perception of a workout's difficulty level. This information provides a more comprehensive view of an athlete's overall well-being.