A training system is a reusable template that determines how a training is structured. Programs and trainings refer to such a system, which then serves as the basis for what gets generated automatically.
Circuit training is a specific system that alternates between 4 exercises or pieces of equipment. Because this quickly ties up 4 pieces of equipment at once — which isn't always desirable in a shared gym (with non-members) or a busy fitness club — you as a coach can exclude specific exercises or equipment from the circuit system.
This way you avoid automatic generation picking exactly the combination that takes up too much space or equipment.
When setting up exclusions you can search and filter by category, and toggle per exercise whether it's allowed to appear in circuit trainings.