Maintenance of standards language is typically under each area's supplement; it basically codifies some forms of past practice. Essentially, it prevents the company from reducing working conditions/work rules/wages/benefits/etc. without negotiating with the local union. There's usually some boilerplate language along the lines of, "The Employer agrees that all conditions relating to wages, hours of work, and general working conditions, as negotiated or agreed upon, shall be maintained at not less than the highest standards at the time of the signing of this Agreement."
It's not a silver bullet, but it can be used creatively to fight back against UPS doing things like taking away cell phone privileges. It's also rarely used because such a case would almost certainly deadlock at a panel and probably require arbitration between the local and company, which is incredibly expensive and time consuming.