so i take it this is for unreal 3 then. Well then this spec mask is probably going to go into your spec power slot and your spec mask will go into your spec map slot.
Save the tga with the spec map in the alpha, then in 3dsMax, put the tga in the diffuse slot and in the specular map slot, then in the bitmap parameters for the spec map slot, change the mono channel output to be "alpha". Like Rick said its just an easy way to save file/memory space by including it in the alpha of one…