On our site an most other, the terms Upgrade, Mod, and accessory are used interchangeably. For our articles here we're trying to be as exact and educational as possible. So we'll use these definition here:
Upgrade: An item that you add to your paintball gun to improve it's performance.
Accessory: An item that you add to your paintball gun to change it's looks but has no impact on performance.
Mod: The physical modification of a stock or aftermarket part, not a new product.
A new barrel would be an upgrade. A shroud would be an accessory. A shoulder stock is somewhere in between. Porting your own barrel to change the sound signature would be a mod.