the difference is that a professional service agreement is a one-shot deal, while a subcontract most often involves an ongoing relationship and a greater degree of involvement in the business. it’s best to fully understand the scope and the technicalities of a professional services agreement, because once both parties sign, it becomes a legally binding contract. a psa has a limited scope and is suitable when project deliverables either require no maintenance or when the skill sets of current employees are sufficient to maintain results once the project is finished. the agreement itself can be long and full of legalese, including an average of about 10 individual sections. items addressed in a psa most often start with a description of the service or project and change order procedures.