TraceOfOne is an option for TR and DiracTrace. Its setting determines the value of the unit trace.
TraceOfOne
TR
DiracTrace
Overview, DiracSimplify, DiracTrace.
DiracTrace[1] DiracSimplify[%]
tr(1)\text{tr}(1)tr(1)
444
DiracTrace[1, TraceOfOne -> tr1] DiracSimplify[%]
tr1\text{tr1}tr1