This illustration shows the Fixing with a PlacementPoint and MeasurementPoint as a PartOccurrance.
The ability to place a Fixing on a specific point in the topology is similar to Grommets covered by generic mechanism of PlacementPoint and PlacementPointReference. Additionally the measurement of Fixing is covered by MeasurementPoint and MeasurementPointReference.
The diagram illustrates the definition of a Fixing with CableTies as Accessory. The upper half of the diagram is the definition of the part master data.
The Fixing is described with a PartVersion and a FixingSpecification. To describe its accessories it has GeneralTechnicalPartSpecification with PartRelations to link the accessories. In this case, one CableTie is mandatory, a second one is an optional add on. Both are referencing the PartVersion of the CableTie.
The CableTie is currently defined with a not further detailed PartOrUsageRelatedSpecification, since there is no CableTieSpecification in the VEC at the moment. That the accessory is a CableTie is defined by the value of the specialPartType attribute.
If there are any additional properties necessary for the CableTie, then they could be specified with CustomProperties for the PartOrUsageRelatedSpecification.
For the instancing of these components, both are created with a PartOccurrence. The Fixing is defined with a FixingRole, the CableTie with a SpecificRole (For the same reasons why a PartOrUsageRelatedSpecification has been used).