Human weapons and suits are unique in that many of them can be installed with addons to improve their characteristics.
Weapon addon composition can be done in the Store screen. It is impossible to change weapon composition in a mission, and composed weapons need to be found in the special equipment section, instead of normal sections.
Suits also have addons, but they are assigned like normal equipments and do not need to be composed.
Composed weapons keep the same size but will be slightly heavier.