MTLRD[p,mu,n,nb] denotes the perpendicular component in the lightcone decomposition of the metric tensor g^{\mu \nu} along the vectors n and nb in D dimensions. It corresponds to g^{\mu \nu}_{\perp}.
If one omits n and nb, the program will use default vectors specified via $FCDefaultLightconeVectorN and $FCDefaultLightconeVectorNB.
Overview, Pair, FVLND, FVLPD, SPLPD, SPLND, SPLRD, MTLPD, MTLND.
MTLRD[\[Mu], \[Nu], n, nb]g^{\mu \nu }{}_{\perp }
MTLRD[\[Mu], \[Nu], n, nb] // FCI // StandardForm
(*Pair[LightConePerpendicularComponent[LorentzIndex[\[Mu], D], Momentum[n, D], Momentum[nb, D]], LightConePerpendicularComponent[LorentzIndex[\[Nu], D], Momentum[n, D], Momentum[nb, D]]]*)MTLRD[\[Mu], \[Nu], n, nb] FVD[p, \[Mu]] // Contractp^{\nu }{}_{\perp }
MTLRD[\[Mu], \[Nu], n, nb] FVLPD[q, \[Mu], n, nb] // Contract0