$MU is the head of dummy indices which may be introduced by Chisholm, Contract, DiracSimplify, FermionSpinSum and various QCD functions. By default it is unset, but can be set to anything.
$MU
Chisholm
Contract
DiracSimplify
FermionSpinSum
Overview, Chisholm, Contract, DiracSimplify.
$MU\text{\$MU}$MU