What is the sixth parameter for fir1

When I use the fdatool to design a HPF, it adds a sixth parameter, 'h' at the end. What is it? It seems to be some kind of flag that allows an even order, (num of coeff), without it fir1 will increment by one.

I have searched the forums and google to no avail. it is undocumented.

Fdatool generated code: b = fir1(N, Fc/(Fs/2), 'high', win, flag, 'h');