Counter Subsystem or Negative Space Subsystem (transparent and non transparent)
Counter/Negative Space Subsystem - What is often referred as a main system, a subsystem defined by the members not already being part of a system with multiple subsystems.
To put the concept in non-system terms, if you were to take every number up to 1000 and put multiples of five in one group and even numbers in another. You still have numbers left over, and those numbers are, in a way grouped together, by not falling in to the other groups.
Obviously you don't have think of things this way, but we do.
Subsolitaire
Subsolitaire - Basically a subsystem but with only one alter.
For example a trauma holder who is separated from the rest of the system, by memory and communication barriers that rest of the system doesn't have as much.