WHAT DOES FEDERAL 243 AMMO 100 GRAIN MEAN?

Wanted some highly rated ammo for my fresh M&P. Bulk Ammo came out on best with the top charges and rapid ,no stress shipping.Most subsonic 9mm ammo with have a lower recoil due to the fact they are usually loaded to some decreased stress (in order to maintain the velocities down below the pace of seem). Also, quite light-weight grain weights have