As mentioned in the example before, there are use cases where it is necessary to define properties of component (instance), without having a concrete part number. This is quite common for example in the electrological design, where for example the cross section area of a wire is specified, but the colouring of the wire is not yet known. In these cases the requirements for component have to be defined without having a concrete PartVersion.
For these cases, the VEC offers the concept of PartUsages. A PartUsage behaves like a PartOccurrence (it has instance specific attributes and can define Roles), but it is not necessary to reference a PartVersion. Instead it is possible to directly reference the needed specifications, which represent the requirements on the concrete component, selected later in the process.