Assuming its the ECC conditions you are using, Subcontractor is defined under Cl.11.2.17 as a person or organisation who has a contract with the (main) Contractor… (this next bit is summarized) and providing parts of the works or services necessary to provide the works.
The boundary however is somewhat muddied when you have labour only subcontractors or a mixture of payment provisions for a singel subcontractor. If a Subcontractor is paid on an output basis i.e unit rates m2/m3, then it falls clearly within the definition of a Subcontractor Cl. 11.2.17. However Subcontractors who are paid for by the Contractor based upon the number of hours they work when they are within the Working Areas are treated as people costs under the schedule of cost components.
Where the costs fall within the definition of people set out in item 14 of the schedule of cost components, their costs should attract the addition of the Working Areas overhead percentage and the direct fee percentage. Where the costs sit outside schedule of costs components but within the definition of Subcontractor they attract the subcontracted fee percentage.
The burden of proof required for payment is a somewhat emotive topic, but the requirements to demonstrate costs varies with the main options (A, B , C & D, etc). Option C for example required the Contractor to meet certains test under Cl.s11.2(23), 11.2(25), 52.2 and 52.3. Generally where the costs are treated as Subcontarctor costs an invoice should suffice. The Project manager may ask to see a coppy of the Subcontarctor conditions under Cl. 26.2.