Defensive Retrofits (Def Rets) are the upgrades that boost the survivability of your ships. They can be used to make your ship stay in the fight longer or keep it alive while disengaging. All of the Defensive Retrofits are viable in different places, so let's see when you should and should not use them:
Advanced Projectors (AP) is a solid card for ships that have a large number of shields. It is not a card seen a lot, but it has its uses. I like it as an option for the second def ret on an assault MC80h. It also has a place on MC30s and other ships with 2-3+ shields per side. Take an ISD for example, it has a shield displacement of 4/3/2 (F/S/R). Typically, with a redirect, you will only be able to take 5-7 damage from one shot before it hits the hull, with AP you can take upwards of 10 damage from one shot without it going through shields. This card makes it really easy to use up all your shields (as you should always try to do) before your ship dies.
Cluster Bombs (CB) is probably the def ret I have seen the least. This does not mean it doesn't have its uses. As the card states, it allows you to essentially counter a single squadron attack even if you die. 5 points is a bit steep, but 4 damage on a squad can sometimes kill it and win you the game. This card is also one of the few cards in the game which, in my opinion, does more to help by just being on your ship than if you actually use it. Think of it as a minor deterrent for your opponent's squads. If there's a chance that you will be able to CB your opponent's squad out of existence then there's a good chance they will reconsider attacking you.
Early Warning System (EWS) is one of my favorite def rets. It allows you to choose one hull zone per round to mark with a chaff token. While that hull zone is defending the attack is treated as obstructed. This can feel like it has little effect on the game but if this procs every round then your saving 6 dice worth of damage from coming at you. On average, 6 dice (red and blue) will do 4.5 damage (6 damage if black). This can also proc multiple times per turn, so there's a chance this blocks a lot more potential damage than just 4.5. Also, it will affect every squadron shot at that hull zone which means if they have one dice and are unable to shoot a different hull zone they will have no shot.
Electronic Countermeasures (ECM) is probably the most popular Def ret in the game. It got a slight nerf at the drop of 1.5 by adding a refresh cost, but it has very little effect on how often it is used. This card essentially allows you to ignore one [accuracy] icon. This can be huge if you're about to take an 8 damage attack and they accuracy your brace. You can just exhaust this card to brace it anyway. Make sure to use this only when you need it because it is only good for once per round. No worse feeling that using this for a big attack just to get hit by a second bigger attack moments later. Don't forget to keep an engineering token around to refresh when you need to.
Reactive Gunnery (RG) is a really interesting card and one that I have been trying to figure out since its release. It allows you to spend any defense token for the salvo tokens effect. This is really handy for ships that do not have access to a salvo token normally. I will also point out that said token must be in the readied state to use this cards effect and let's say you have two redirects, if you spend one for this cards effect you may not spend the other to redirect since you can only spend one of each type of token per attack. This card does exhaust but there is no refresh cost.
Redundant Shields is probably the worst card on this list. Cons: Modification, 8 points, Eats up the precious Def ret slot. Pros: one shield per turn? It is just way too many points to really justify bringing. Other def rets will serve you much better and save much more damage from coming through. If you really need shields just dial up a repair command and go to town. I could see this card being handy if it were not a mod and maybe 5-6 points, but even then it's still maybe mid-tier.
Reinforced Blast Doors (RBD) is a really easy-to-use and solid card. 5 points to discard 3 facedown damage cards once is just good. It's not spectacular but it's pretty damn solid. Do make sure you use this card before you die, if that means you only discard 1 or 2 damage cards so be it, at least you didn't waste 5 points. 5+ hull is the best place for this card, but if you really must take it on 4 hull then it can still do some work. Be careful not to get one-shotted though. This card cannot revive you.
Thermal Shields is the newest addition to this list and the only faction locked def ret. It can only be taken by Republic or Sepertist ships and can only go on medium or large ships. Thermals is a card that has been quite popular since its release. It has had a few points of contention around it but I think it has all been resolved up to this point. You may declare thermals after your opponent has gathered dice but before they roll them. This is a very thin timing and it is easy to forget, just make sure you do it before they roll their dice, or else things can get a bit messy. Spend a brace to halve the attack pool (rounding down). This can be really handy for removing specific colored dice to avoid critical effects or to just avoid damage in general. There are a lot of questions that come with this card and all of them are answered in-depth here, but I will give a couple:
Intel Officer does not work on the brace spent by this card.
You may not brace during the normal window. Each type of defense token may only be spent once per attack.
This card does not affect the addition of dice through "add" effects such as Ackbar, Ex Raxs, or Con Fire. It does affect things like enhanced armament or spinals.
Yes, the defender gets to choose which dice are removed.
If your opponent forgets to use their thermals: be nice, talk it out, if you just rolled and they realized they forgot just roll again. There's a lot to remember in Armada, just be friendly.