FCE[exp] translates exp from the internal
FeynCalc representation to a short form.
FCE is equivalent to FeynCalcExternal.
Overview, FeynCalcExternal, FCI, FeynCalcInternal.
ex = FCE[{DiracGamma[5], DiracGamma[Momentum[p]]}]\left\{\bar{\gamma }^5,\bar{\gamma }\cdot \overline{p}\right\}
ex // StandardForm
(*{GA[5], GS[p]}*)ex = {GA[\[Mu]], GAD[\[Rho]], GS[p], SP[p, q], MT[\[Alpha], \[Beta]], FV[p, \[Mu]]}\left\{\bar{\gamma }^{\mu },\gamma ^{\rho },\bar{\gamma }\cdot \overline{p},\overline{p}\cdot \overline{q},\bar{g}^{\alpha \beta },\overline{p}^{\mu }\right\}
ex // StandardForm
(*{GA[\[Mu]], GAD[\[Rho]], GS[p], SP[p, q], MT[\[Alpha], \[Beta]], FV[p, \[Mu]]}*)ex // FCI // StandardForm
(*{DiracGamma[LorentzIndex[\[Mu]]], DiracGamma[LorentzIndex[\[Rho], D], D], DiracGamma[Momentum[p]], Pair[Momentum[p], Momentum[q]], Pair[LorentzIndex[\[Alpha]], LorentzIndex[\[Beta]]], Pair[LorentzIndex[\[Mu]], Momentum[p]]}*)FCE[ex] // StandardForm
(*{GA[\[Mu]], GAD[\[Rho]], GS[p], SP[p, q], MT[\[Alpha], \[Beta]], FV[p, \[Mu]]}*)