Field help (WST3) 

This field identifies the production resource that is to be used for this operation. This work center is the main (principal) work center for this operation. It can be a machine, labor or subcontract work center. The scheduling of the operations looks at the availability of this work center.

Work centers of type 'subcontracting' are managed using the lead time defined in the subcontract product record. Subcontract operation times are not used by the scheduling.