A sub-group is a group within a group.
Sub-groups allow objects to be associated with other objects, without necessarily taking on all of their characteristics.
For example, the kitchen vignette below contains a dishwasher. You want a customer to be able to select the dishwasher and apply a color...
... or a cabinet front that matches the surrounding cabinets:
If the dishwasher is in a separate sub-group, the customer can choose whether the dishwasher receives a color or adapts the finish of the surrounding cabinets.
In the illustration below, the group Cabinets contains four sub-groups: wall cabinets, base cabinets, refrigerator, and dishwasher.