The Purchasing of items for Jobs is basically no different from that in Business Central.
However, items are usually ordered on an Sales Order or Job basis. For this DYCE Job Billing offers a simplification compared to the standard of Business Central. Further explanations on how to create Job-related purchase orders can be found in this part of the DYCE documentation.
In Role Center Project Administrator there are two tiles Purchase Orders required and Accrual required under the heading Purchase Orders. These indicate whether purchase orders (based on time recordings) or accruals (based on an outstanding purchase invoice) still need to be created for external resources. For more details on purchasing external resources, see this part of the DYCE documentation.