This is where a list of teachers who have less classes allocated to them (than their assigned teaching load) can be found.

In turn, it effectively lets you find existing staff who might be able to cover a lesson; instead of you having to hire a casual.

A teaching load is basically a measure of how many classes a teacher is prepared to take; therefore they'll have a certain number of 'free' periods in their ordinary timetable.

Data for this is often in yuor timetable package and handled in Edval