FCGPL[{inds_}, var] represents a Goncharov polylogarithm (GPL) with indices inds and argument var.
FCGPL[{inds_}, var]
inds
var
As of know this symbol merely acts a placeholder and does not implement any mathematical properties of GPLs.
Overview, FCHPL
FCGPL[{1}, x]
G(1; x)