Subgroup

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A subgroup is a group of system members within a group.

These system member stick together and spend time with each other, sometimes almost exclusively, more than they do with the larger group or system. They may often front together, and they might live together or near each other in headspace.

It can also serve as an umbrella term for any situation of multiple headmates being more closely connected to each other than in a larger group, or those more cut off from the rest of the larger group / system / sisasystem.

Related Terms
The term subsystem is listed to be able to function as an umbrella of sorts for subgroups within a system.

Clusters are groups of headmates who are grouped together based off some sort of similarity.