Different between Subcontract and Purchase Service.

Hi all

I want to know about difference between Subcontract and Purchase Service.

My company implementation a Construction Project and a Production Company.

In Construction Project:

My company will be supplied main-material, Vendor supply labor and sub-material.

Contract between my company and Vendor is payment when complete.

In Production Company:

My company will be supplied main-material, Vendor procedure finish goods. My company will receipt finish goods.

I don't know which is Purchase Service, which is Subcontract.

Regards,