My Square D info says that the add-on bar is for equipment grounding ONLY. The factory installed neutral bar(s) are required to have a minimum number of terminals for landing neutrals based on the number of circuits the panel is arranged to supply. Where there are a large number of equipment ground conductors they MAY be landed more than one to a terminal IF the panel is so listed (and marked) OR an additional bar kit must be installed. Terminals used for equipment grounding are treated differently than those for neutrals because the connection does not undergo the cyclic loading and expansion/contraction of the current carrying neutral.
