TraceOfOne is an option for TR and
DiracTrace. Its setting determines the value of the unit
trace.
Overview, DiracSimplify, DiracTrace.
DiracTrace[1]
DiracSimplify[%]\text{tr}(1)
4
DiracTrace[1, TraceOfOne -> tr1]
DiracSimplify[%]\text{tr}(1)
\text{tr1}