When isSAGBI is supplied a SAGBIBasis $SB$, then this flag determines whether the sagbi generators or the subring generators are checked for being a subalgebra basis. Explicitly, if the flag is set to false then the sagbi generators of $SB$ are checked for being a subalgebra basis. Otherwise, if the flag is set to true then the generators of the underlying subring associated to $SB$ are checked for being a subalgebra basis.
The object UseSubringGens is a symbol.