Defines how the set of acessoryParts referenced by a PartRelation should be interpreted.
Mandatory means that in a usage of a component all referenced acessoryParts must be used.
Optional means that in a usage of a component some referenced acessoryParts can be used by choice.
OneOfAll means that in a usage of a component exactly one of the referenced acessoryParts must be selected.
Custom means, that there is some kind of custom constraints / logic between referenced PartVersions that has to be evaluated to determine the selected accessories.