With the PartRelations other PartVersions (those) can be defined as supplementary parts for the PartVersions that are described by the containing GeneralTechnicalPartSpecification (this).
The semantic is, that if this PartVersion shall be used in a correct way in a product, those referenced PartVersions can or shall be added to the product as well (depending on the used PartRelationType). That can be for example caps, bars or levers in case of a connector.
The PartRelation.relationType defines how the set of referenced PartVersions of one PartRelation must be interpreted. For details see the description of class PartRelation.
The association via the PartRelation just defines what other components may be used together with this component. It does not further specify the location of the usage or its function. However, in some cases a more detailed specification of the location is needed, e.g. if the supplementary part is required for a specific slot in a connector housing. In such a case, the PartRelation is specified by the GeneralTechnicalPartSpecification and may be referenced by its specific usage location (currently from a Slot, ModularSlot, ExtensionSlot or a PlacementPoint).