FeatureOfInterest should be systems or parts of it
Clause 5.3.3.2 states that "Each subclass of saref:FeatureOfInterest should also be defined as a subclass of s4syst:System, s4syst:Connection, or s4syst:ConnectionPoint."
It is not clear which requirements lead to this restriction or if this comes from some extension.
It entails that any feature is a system of part of it which may not be the case.