The Federal Government primarily categorizes its acquisitions as supplies, services (other than research and development), research and development (R&D), and construction. The Government acquires services because it has chosen to rely on contractors to do much of the work it must do for the American people. According to the Federal Procurement Data System (FPDS), services accounted for the largest categorical share of the Government’s contract obligations in Fiscal Year 2023, and professional services other than research and development accounted for the largest single share of service obligations ($108.6 billion).1 But while the Federal Acquisition Regulation (FAR) provides general definitions of supplies, construction, applied research, and development,2 it does not provide a general definition of services.

See Contracting For Services: Part I—Conceptual Definition And Model by Vernon J. Edwards

Excellent paper!  It really made me think on how to better communicate work statements effectively.  The Guidelines at the end are so useful and especially this one:

8. Remember that effort is not product. Value lies in the change the contractor produces in the service object, not in the effort made to produce the change.