What criteria does ABRSM use in assigning piano music to each grade?

For each piano exam grade, ABRSM publishes a book of music curated for that grade. In addition, they provide references to other pieces acceptable for that grade-level exam.

What criteria are used to curate the syllabus at each grade?

For a piano teacher (or student) looking to further supplement learning at a particular level it seems knowing these criteria would be very useful.

There are, of course, a variety of grading systems for piano music. The RCM has theirs, and there is also Jane Macgrath’s standard Pianist’s Guide to Standard Teaching and Performance Literature. But I’m specifically interested in the ABRSM criteria.

---

Related question: Help with understanding ABRSM grades?