CovariantD[mu] is a generic covariant derivative with
Lorentz index \mu.
CovariantD[x, mu] is a generic covariant derivative with
respect to x^{\mu }.
CovariantD[mu, a, b] is a covariant derivative for a
bosonic field that acts on QuantumField[f, {}, {a, b}],
where f is some field name and a and
b are two SU(N) indices in
the adjoint representation.
CovariantD[\[Mu]]D_{\mu }
CovariantD[\[Mu], a, b]D_{\mu }^{ab}
CovariantD[\[Mu], Explicit -> True]\vec{\partial }_{\mu }-i g_s T^{\text{c11}}.A_{\mu }^{\text{c11}}
CovariantD[\[Mu], a, b, Explicit -> True] // StandardForm
(*RightPartialD[LorentzIndex[\[Mu]]] SUNDelta[a, b] - QuantumField[GaugeField, LorentzIndex[\[Mu]], SUNIndex[c12]] SMP["g_s"] SUNF[a, b, c12]*)