The M9 has been modified as the M9A1, adding, among other things, a Picatinny rail for the attachment of lights, lasers, and other accessories to the weapon.
The M9 has been modified as the M9A1. The M9A1 is adding to the weapon. This is among other things. Other things are a Picatinny rail for the attachment of lights. Other things are a Picatinny rail for the attachment of lasers. Other things are a Picatinny rail for the attachment of other accessories.