If a work center is a subcontracting work center, you can select a supplier for that work center here. This supplier will then be selected by default when creating a preparation for an operation used by this work center in Preparation, even though you are allowed to change to another supplier in that procedure.
When printing manufacturing order documents, you can also print subcontract purchase orders and possible subcontract delivery notes to the supplier selected on the operation row. This requires that these documents have been defined in the Form Settings procedure under Global Settings, and that you have checked the setting Print work card (see below).
When reporting a subcontract it is also possible to create a supplier invoice basis. This is determined by the setting Create supplier invoice basis during subcontract reporting?, under the Accounts Payable heading under the Purchase tab in the Settings procedure.