You can use sub-consultants in Abtrac if you outsource work to third parties, and need to manage them over the duration of one or more projects.
They may submit invoices for you to authorise and pass on directly to your client for payment. Alternately they may submit invoices to you which you pay and then you invoice your client as if the sub-consultant is part of your team. At that stage you may also apply a margin to the sub-consultant invoice.
The essential feature is they are submitting their claims as a series of creditor invoices as they work through their contract requirements on projects for your clients. You need to know what they have already invoiced, what is this particular invoice for, and how does it fit within the contract agreements you have with them.