According to FAR 37.102(e), "To the maximum extent practicable, the program officials shall describe the need to be filled using performance-based acquisition methods." According to the Defense Acquisition Guide, Chapter 10, the PWS and SOW are considered two different things. In Section 126.96.36.199.1, it states "there is no mandatory format for the PWS or the SOO. Follow your agency’s procedures for the proper format required for these two documents." In a PWS, the Agency should define the results to be achieved, and solicit a wide variety of technical solutions from contractors in order to empower the private sector to craft innovative solutions rather than stifle that ability. Unlike a SOW, a PWS should describe the outcomes (or results) rather than how to do the work.
Normally the Government writes the PWS based on the Government's high level objectives and desired results, but if the Government use a SOO in lieu of a PWS, the offeror(s) can propose a PWS which the Government will then need to negotiate with the offeror(s). Also, it's not always possible to write the PWS tasks strictly in terms of desired outcomes and commercial standards. In DoD, it's not unusual, due to agency regulations and military standards, to have to specify some of the required tasks more tightly and to have to tell the offerors how they must accomplish some of the tasks in the PWS.
In these type of situations, the Multi-functional Team (MFT) is still expected to minimize as much as possible specifying tasks (and only as required by regulations or law) and use performance-based requirements as much as possible in the PWS. If the PWS is written using performance-based methods as much as possible, then it's still considered a PWS even though parts of it were specified as required by law or regulation.