In accordance with DoDI 1015.15, each of the Military Service and exchange treasury oversight group "shall determine the benchmark that will be used to evaluate portfolio performance" for each calendar year.  "The benchmark shall be unambiguous, measurable, and appropriate." 

Examples of indices authorized to create the portfolio benchmark are included in the PDF document here:
DoDI 1015.15 Benchmark indices

Per DoDI 1015.15, the benchmark "shall reflect the broad risk and return characteristics of the authorized portfolio holdings.  A composite benchmark may be comprised of several indices provided the names and weights of each index are clearly defined.  At year end, the benchmark return shall be compared to the portfolio average annual yield and deviations evaluated based on investment maturity, credit factors, and investment instruments."