I would like to note that the Section 2.1 of our User Agreement speaks to subcontracting on fixed price contracts:
"You further acknowledge, agree, and understand that ... unless otherwise agreed with their Client, Freelancers may use subcontractors or employees to perform Freelancer Services by delegating work on fixed-price contracts or by agreeing with their Clients to have hourly contracts for Freelancer’s subcontractor(s) or employee(s)."
It's best practice for the client and the freelancer to discuss any terms and arrangements before the contract starts, including whether the freelancer is planning to subcontract and whether the client disallows subcontracting on the contract.