Most of the weapons in the game have modification slots (blue weapons and the better ones - purple and gold). If you want to check them you must first select a piece of equipment and see its details. In this screen you can see currently used modifications, some weapons can have already installed modifications.
Usually, weapon modifications are divided into 3 categories:
- Type of attack in an energy weapons,
- Attack modifications (kinetic and exotic weapons),
- Weapon colors (paintjobs), so called, Shaders,
- Gold weapons can also have Ornaments which are also pure cosmetics.
You don't have many options when it comes to weapon modifications, they are purely cosmetic. But if you found a weapon that you want to use for a longer time, changing its appearance can make the game more pleasant or give you prestige among other players. Most of the legendary weapons have a modification that increases damage, however, you can't detach them - only change. Sometimes, if you have a free slot you can install a modification that slightly increases weapon's stats (e.g. stability) but you can rarely increase damage.
You should also notice that most weapons, starting with blue ones, usually offers you an option to change their perks. Switching between them is free and you can do it as many times as you want. When you do that you slightly change a weapon's stats which can be noticed on bars in the bottom right corner. If a weapon has scopes you can switch between them and you can feel the difference during the gameplay - because of that you can decide to use a scope that is better for you rather than focusing solely on stats.
Armor modifications go one step further. Here, besides aesthetics elements (which are the same: Shaders and Ornaments - they are more visible on other players) you have more opportunities with installing modifications into free slots. Notice that almost each part of your equipment can be modified (as long as that item is, at least, blue). Most of the times, modifications doesn't affect the quality of defense given by a given part. Usually, they increase their stats, e.g. increased health regeneration and even shorter cooldown for grenades or better stability for certain types of weapons.
Usually, Exotic and legendary equipment can be Infused. This operation can be done once for each item and there is no option for reverting to the previous state. Infusion allows you to absorb Power of other item (it must be of the same type: if it is a helmet then you must offer another helmet). This allows you to use a weapon that has additional perks by a longer time - you don't have to stop using a weapon with special stats and switch into a more common one that adds more to Power modifier.
If you want to Infuse a weapon/armor you simply have to see their details in your inventory and select black-gold Infusion icon. You must have a second item that you want to sacrifice and transfer its Power. An item that was used in this process is then absorbed and disappears from your inventory.
Mods are encountered rather rarely. You can find them in chests, as quest rewards, or contained in White Engrams. Nevertheless, the easiest way of acquiring modifications is by improving your reputation at the gunsmith and by decrypting the engrams received from them. To improve your reputation, bring weapon parts that are found when dismantling blue items.
You should have a hefty supply by the end of the story campaign. The upgrades are divided into classes (normal - blue, or legendary - violet) Remember that an installed mod cannot be removed - replacing a mod means the destruction of the previous one. Moreover, avoid dismantling blue items, as these yield only migot. Dismantling violet items yield additional gunsmith materials which can be used to buy other mods from the gunsmith (with other parameters or different types of equipment).
Tip: A 280 level character can replace old and cheap mods with new, legendary ones. The new mods are generated randomly, but now you can make use of blue mods by replacing them with legendary ones.