When can Invoices be added into a Service Request?

Confused about how the Invoicing system works? 

If a Purchase Order (PO) Number has been assigned prior to completion by the Service Professional (External Vendor), the Invoice can be created/attached at any time following, even after the request has been archived. NetVendor Maintenance was designed so that the vendor invoicing process will not hold up the natural flow of the Service Request.

This image below should help describe when an invoice can be created and what actions are possible by a Property Manager or Service Professional user role.