Commissioning, including acceptance testing, is a somewhat rigid process intended to verify and validate that a project delivers what is required and expected. As with most processes it is paramount that each step, phase, or activity is begun and completed sequentially to keep the process flowing smoothly. At a high level, this is obvious. You shouldn’t start the design phase until the programming and requirements definition phase is complete and you shouldn’t start construction until the design is complete, etc.
This strategy holds true for acceptance testing as well. Acceptance testing should progress from factory testing, to equipment receipt and installation and continue through startup, followed by performance testing, and culminate in integrated systems testing (IST). Furthermore, the testing should start with verifying the simple before proceeding to the complex. Start with verifying components, then equipment followed by systems and finally culminating with IST.