A HubGroup is not an element in its own right, but rather a subset of the rig used for organizing rig components. A HubGroup comprises a hub, its limbs, and any tails.
You see HubGroups wherever the CATRig hierarchy appears, such as the CATMotion Editor. The HubGroup name starts with the name of the hub followed by “Group”; for example, RibcageGroup.
The PelvisGroup on the Panther rig