Subs do most of the work on a commercial building. Each one bids its trade to several general contractors that are bidding the same job.
Electrical, plumbing, drywall, flooring, roofing and gym equipment are typical sub trades. A sub can hire its own lower-tier subs and suppliers.
As a sub, you price from the same bid packet as the GC. You are bound by the terms that flow down from the prime contract.
Read the front end documents, not just your spec section. Bonds, wage rules, insurance and retainage can all reach you.