FAR 37.000 "...requires the use of performance-based acquisition (PBA) for services to the maximum extent practicable." Your requirement is for IT services, so we must also consult FAR part 39. FAR 39.104 states: "...unless the contracting officer determines that the needs of the agency...requires the use of other than (emphasis added) a performance-based acquisition." In other words, PBA should be used when possible, based on the specifics of the acquisition. If the contracting officer determines PBA is appropriate, the solicitation must include a performance work statement or statement of objectives (SOO). If a SOO is used, the contractor responds with its own PWS. Please thoroughly review FAR subpart 37.6 for full details.