Fittings are an important aspect of the equipment system. At the most basic level Fittings are similar to gems or runes in other games that are placed in a "socketed" item. You are easily able to change the socket types on a piece of equipment.
Weapons and Armor should be considered shells. They provide the visual appearance and the basic type for that piece of equipment. Fittings allow you to customize those shells and are used extensively. They allow for a player to customize a weapon for a certain situation (say versus a robot or an enemy with heavy armor), add special effects, or just create a standard issue jack of all trades weapon. Through these combinations a weapon can have thousands of different possibilities. Most fittings can be removed and sold on auction. Some will be destroyed on removal, though, and others will be consumed after a certain amount of use.
Most weapons and armor have five Fittings slots. Each fitting slot has a selectable socket type, which can be customized by the player. There is a checks and balances system in place to prevent players from stacking too many of the same types of fittings and unbalancing the system. Each fitting is tied into a certain armor or weapon skill and will require a certain level of skill to be used.
This setup allows for a variety of fittings to be added to your weapon and you get to choose how you want to structure the weapon. You can make it balanced, making it equally versatile against all three armor types (light, medium, and heavy), or you can make it unbalanced and stronger against one of the armor types and weaker against the other two. These types of fittings occupy the "A" style fittings and you can place two of them on a weapon. The other two are reserved for the other style of fittings that can affect the handgun such as accuracy, range, durability, critical chance/multiplier, health, attack rate, procs, damage, penetration, model visuals, etc. Each of these fittings are a certain letter type and you can only use one of those letter types at a time on the handgun.
Most fittings are created through the crafting process and they are the ultimate result of multiple recipes that affect the final quality of the fitting. Using superior ingredients will produce a superior fitting. Each fitting will have a Tier value (used to restrict it to a certain skill mastery) and then a grade and sub-quality value. All three values modify the end result strength of the fitting and applied together create 500 variations of the values based on the progression from Tier 0 to Tier 9. The crafting system also supports permutations of the end result based on the quality so it is possible for a recipe to produce a different fitting based on the grade and tier of the end result.
Also of importance is the ability for most fittings to be removed and the capabilities for fittings to be consumed. Most fittings in the game will be able to be removed from the socket and can be exchanged or traded with another player through the auction system. Fittings will also decay through usage and eventually drop in grade.
You will have the option of using up a fitting slot to slow this process down. In addition, some fittings will support a quicker rate of item degradation in return for slightly elevated stats.