Everything a droid does other than store itself and move is determined by what droid modules are installed in it…

Everything a droid does other than store itself and move is determined by what droid modules are installed in it. Droid modules are installed in sockets. Each type of droid chassis (R2, R3, R4, R5, Probot, Protocol, and so forth) has a different number of each of these types of sockets. Any particular droid chassis can have two of three of these different types of sockets:

• Combat Module Sockets.  Droid Combat Modules can only be installed in these sockets, but any other module except droid armor modules can be installed in these sockets as well. A droid that has Combat Module Sockets will never have General Module Sockets.
• General Module Sockets.  Any module can be installed in these sockets except Droid Combat Modules and Droid Armor Modules. A droid that has General Module Sockets will never have Combat Module Sockets and cannot be built as a combat droid.
• Armor Module Sockets.  Only Droid Armor Modules can be installed in these sockets. The more sockets and the higher the quality of the armor modules installed, the higher the "health" of the droid and the more damage it can take in combat. Not all droid chassis have armor module sockets. Non-combat droids can't fight and and can't be attacked (except maybe in player-vs.-player), so armoring them is a waste of resources.

What Droid Modules Are There?

Here is a list of every droid module there is, along with a description of each:

• Droid Armor Modules.  There are six different kinds, from Level One to Level Six, with Level 6 Droid Armor being the best. The more and the higher the level of the armor modules installed, the more damage a droid can take in combat. As stated above, Armor Modules can only be installed in Armor Module Sockets and not all droids have these sockets.
• Droid Combat Module.  There's only one kind, and the quality of the resources and experimentation done on this module directly affects how much damage each module adds in combat.  The more of these modules are installed (and the better their quality), the more damage a droid can do, so it makes sense to specialize combat droids so all available Combat Module Sockets are filled with these modules. As previously stated, only droids with Combat Module Sockets can use these modules and become combat droids.
• Crafting Modules.  Five of these exist, one for each specialized crafting tool (Clothing and Armor, Food and Chemical, Starship, Structure and Furniture, and Weapon, Droid and General Item). When a droid with one of these modules is next to a character using a matching specialized crafting tool, the tool acts as if it was near a private crafting station with no Functionality Rating. Since a stationary private crafting station (built by a Trader in Structures) can have a FR that approaches 45, the best work is done with those.  Still, a crafting droid can let a Trader build anything anywhere, even on board a Player On Board ship (if they ever fix it so droids can be called indoors again) or in the Dathomir wilderness or the streets of Theed, so it can really come in handy.
• Droid Item Storage Modules.  Six different types of these exist, from Level One to Level Six.  A single Level 6 Droid Item Storage Module, properly made, can store ten items, the maximum for any droid no matter how many of these modules are installed.  When examining a droid deed, a Storage Rating of 12 guarantees that the droid can hold ten items.  To access this inventory space, call the droid and open its radial menu and look in the Droid Options menu.
• Droid Data Modules.  Six different types of these exist, from Level One to Level Six. A data module allows a droid to do two things: act as an Astromech on certain starfighters (such as the X-Wing and Y-Wing) and store extra manufacturing schematics for use in factories. The level of the module determines the level of Astromech and the number of schematics the droid can hold. A properly made Level 6 Data Module can hold 150 extra schematics, but only an Alliance ace pilot can use such a droid as an Astromech.
• Effects Modules.  These modules are used only by Entertainers to generate special effects during their performances.  Six of these exist, each with a different effect: Avian, Confetti, Dancing Jawa, Electric Fog, Foam and Mind Bloom. (Be sure to specify when ordering.)
• Playback Module.  Each of these can be programmed only by an Entertainer with Musician skills to play background accompaniment music to enrich his or her performance.  The type of music played is selected by the Entertainer and can be changed later.
• Merchant Barker Module.  A droid with this module can be programmed by a Trader with the Advertising IV skill to speak a short phrase (128 character or less) whenever a player's character approaches within twenty meters.  The droid can also store a waypoint, and any player may use the droid's radial menu to copy this waypoint to his or her Datapad.
• Droid Detonation Module.  Makes the droid explode on command. The more of these modules are installed and the better the resources and experimentation used to build them, the more damaging the explosion. The droid, of course, does not survive the blast. Only Bounty Hunters and Smugglers can detonate these droids.
• Maintenance Modules.  Allows a droid to add credits to the maintenance pool of one or more harvesters.  No, such droids can't add power or retrieve resources from harvesters, so these aren't so useful.  Three different types of these exist, from Level One to Level Three. A single Level 3 Maintenance Module, properly built, can be programmed service up to nine harvesters at any one time.  Adding more modules will not allow a droid to maintain more than nine harvesters.  To program this module, you call the droid after you place a harvester, then open the harvester's radial menu to assign the droid to that harvester.  (A new menu option will appear in the Maintenance menu.)  Later, you can call the droid anywhere outside and tell it to deposit credits to one or more of the harvesters it has been assigned to service.  The droid will show you a list with all of the harvesters it has been assigned, along with the credits in each of their maintenance pools (which the only worthwhile feature), and you pick the harvester to send the droid to and how many credits to deposit. After you have done this for any or all of the harvesters, you send the droid on its way and it will be unavailable for an hour or two (it will say how long) as it disappears to buy interplanetary shuttle tickets (which you pay for separately and automatically) to get to the planet(s) the harvesters are on and adds the credits.
• Creature Harvesting Module.  Broken with the NGE, fixed in Chapter 3.6.  These modules allow a droid to harvest meat, bone or hide from a creature that you or your group kills. Like Droid Combat Modules, the number and quality of modules used affect the efficiency of the harvesting, so the best creature harvesting droids are Advanced R3 droids with six of these. Such droids can increase harvest yield by 70% or more. They can harvest on command or be set to automatically harvest a creature as soon as its corpse hits the ground.
• Trap Dispenser Module.  Broken.  Scouts and Rangers used these and those classes were deleted by the NGE.
• Medical Modules.  Broken.
• Stimpack Dispenser Module.  Broken.
• Droid Repair Module.  Not broken, but it's only usable by Traders in Engineering to fix wounds on droids, and no creature, whether a player character, creature, NPC, droid or other pet takes wounds since the NGE.  Hence, this module is useless.