From Pluralpedia, the collaborative plurality dictionary
compound (n., adj.)
Applies tosystems, headmates, system functions
Coiner HerbPhoenix

Compound is a term used to describe when system members or sisasystems blend together to create variations of a whole.

This could mean that two members blend together and based on which one is more prominent, the combined form changes. The same could go for sisasystems.

There is also no limit to the combinations and how many members or systems can be part of the compound.