(subcomponent-of ?sub ?super) means that the component ?sub
is structurally a part of component ?super. A component cannot be a subcomponent of itself (irrreflexivity) and two components cannot be subcomponents of each other (antisymmetrity).
This is a primitive relation. Formally, any irreflexive and antisymmetric relation defined only on components will satisfy this definition.
Note that the subcomponent-of relation does not name the parts of a component. That is, the subcomponent links are unlabeled arcs. For named subcomponent relations, see has-subpart-slot.
A main difference betweeen components and arbitrary globs of matter is that components are object-like in a modeling sense; thus, a subcomponent is not an arbtrary subregion, but a part of a system with a stable identity as its part. If subcomponent-of were transitive, then there would be no level boundaries between a component and its subcomponents and their subcomponents; for modularity reasons, the system modeler describes the subcomponents of a component as black boxes, rather than as arbitrary regions.
Mereological theories are concerned with the validity of axiomatic assumptions about part/whole relationships as applied to the physical world. That is why they often insist that the part relation is transitive. This ontology provides abstractions for engineering modeling; it is the modeler's obligation to apply them meaningfully to physical domains.