Sub amenities are amenities that are children of other amenities. If a reservation is made on a sub amenity, the parent amenity is also reserved. However, the other sub amenities will not have a reservation. If a reservation is made on the parent amenity, all of the sub amenities will be reserved.
For example, if you have 2 pickleball courts on a tennis court, you can setup pickleball court 1 and pickle ball court 2 as sub amenities to tennis court 1. If a reservation is made to one of the pickleball courts, tennis court 1 will not be available to reserve during that time, but the other pickleball court will still be available. If a reservation is made for tennis court 1, neither of the pickleball courts will be available during that reservation time.
You can create a sub amenity by clicking the + sign next to the parent amenity in the Club Amenities section. Note that sub amenities are treated as amenities from a subscription perspective.