$FCDefaultLightconeVectorN is a global variable which is set to FCGV[ānā]. It denotes the default name for the vector nnn in the lightcone decomposition for Lorentz tensors.
$FCDefaultLightconeVectorN
Overview, MemSet, FCMemoryAvailable.
FCGV(n)\text{FCGV}(\text{n})FCGV(n)