UxP is an umbrella
It encompasses several obvious disciplines, which have some overlap.
UxP has a close working relationship with Documentation. This area is vital to our success because it is the vehicle by which we communicate. UxP is often the project's de facto documentation manager.
UxP has a close working relationship with QA, because UxP is the ultimate owner of product quality.
UxP has a close working relationship with Process Management. One of the biggest challenges in product development is reiterative design - the "fine tuning" of a service in response to customer, client and enterprise needs.