Each move in your repertoire is linked to some information on when you last reviewed it, how difficult it was, your review history, etc. We use this to determine when you should next see that move, to ensure you're seeing the moves you're likely to forget soon, but not seeing moves that are too easy for you and getting overwhelmed. Our spaced repetition system is based off of [FSRS](https://github.com/open-spaced-repetition/fsrs4anki/wiki/ABC-of-FSRS). It is inarguably the best open-source spaced repetition system out there, and arguably the best spaced repetition system in general. Move progress is shared between repertoires, so if you play `1.e4` in three different White repertoires, that progress will be shared.