As a general rule, the competition requirements set forth in FAR Part 6 do not govern how prime contractors procure supplies or services from subcontractors. I do know, however, that prime contractors often voluntarily adopt the rules of FAR Part 6 for purposes of procuring goods and services from subcontractors or suppliers in order to obtain Government approval of their purchasing systems.
I also know that prime contractors often voluntarily adopt FAR Part 6 competition requirement to satisfy the requirements of FAR 52.244-5 which provides that prime contractors “shall select subcontractors (including suppliers) on a competitive basis to the maximum extent” when providing goods or services under a contract that exceeds the simplified acquisition threshold and is not a firm fixed price contract awarded on the basis of adequate completion or prices are set by law or regulation or a time and materials, labor hour or architect/engineer contract.