Set groups are used if you teach pupils in ability group. So, for example, you may have a group called Maths Set 1 and another called Maths Set 2. You can filter these pupil groups into the OTrack assessment screens and in OTrack reports and analysis.
Custom groups are similar to contextual groups such as FSM, pupil premium and EAL etc. So, for example, you may have a group that identifies pupils that live in a particular area of your catchment or who have had access to an intervention. You can filter these pupil groups into OTrack reports and analysis only.