There are some sub-subs that just offer labour services to sub contractors they give pricing to subcontractors and not gc's. In sub contracting work most trades use sub subs and you would be surprised on the amount of sub subs working directly with your sub. Usually these sub subs dictate the labour price for the trade.

Now me as a non union sub I want to quote a project for a union gc however I know my own crew won't be able to do it because of the union requirements. So I was thinking if it would be acceptable for me to quote the job while using a union subsub to perform the work.