The 100 in 100 is a maximum allowed openings before the wall would be compromised. If the wall will have metal boxes larger than 16 square inches, every one of the large ones would have to be protected with putty pack or boxed in with fire rated GWB. OR, if there were more than 100 square inches of accumulated area of metal boxes in any 100 square foot of wall, then you would have to putty pack or box in those boxes in excess of 100 square inches. Last, if you had to place metal boxes back to back within 24 inches, then you would have to putty pack one of them.
