This category lists several tens of skills whose common feature is that they are useful for silent elimination of the enemies, both single ones, encountered during the free exploration of the game world, and the more organized groups that stay in the predator rooms. This category includes, first of all, the upgrades for Batman's standard gadgets, which make them more efficient (e.g. higher number of uses, increased range), as well as add new functions (e.g. mine deployment or in-built proximity detonator). You can also find here a few other items, like a very helpful attack from above.
Unlike the first of the categories, in this case, you need to unlock individual upgrades thanks to the upgrade points that you obtain so, I recommend that you get acquainted with the hints below, which will tell you which upgrades are the most useful. Note that the whole thing has the outlay of a skill tree so, if you want to unlock the best upgrades, you need to purchase several other first.
The initial upgrades are, to be honest, more useful in regular combat, because this gadget is not too useful for silent elimination of enemies. With time, you should turn your interest to the Triple Batarang, though, because it is required to solve some of the puzzles connected with Enigma's collectibles.
All three upgrades which are not connected with any of the gadgets, are very useful. The Glide Boost Attack is a good way to start a fight, because you can, this way, eliminate one of the bandits from the fight. When it comes to the Heat Signature Concealment, it is a good idea to purchase this upgrade right after you encounter the first enemies equipped with thermal vision, thanks to which you will not have to worry about being detected while staying in a vantage point. The Quick Silent Takedown allows you to cut down the time required for a takedown, thanks to which you decrease the risk of being spotted by an enemy.
Just as useful are the upgrades to the glue grenades. I especially mean here the Glue Frag Grenade, thanks to which you can stop several enemies at a time and eliminate them, or their colleagues in the other part of the location that you are currently securing. Another gadget that you will be using frequently is the explosive gel, although here I recommend the Explosive Gel - Proximity detonator only, thanks to which you will not have to wait for the right moment to detonate the charges.
When it comes to the upgrades connected with the grapple, it is a good idea to unlock the Grapple - additional line and Grapple - Increased Hoist Capacity. Using the grapple will allow you, at the outset, to clear the predator rooms and eliminate several enemies without any problems, either by pulling gas tanks towards them, or by hanging them up to vantage points (that is what the cord is for).
For the end, I recommend, you should leave upgrading the smoke pellets and the disruptor. Paradoxically, the best way in which the disruptor can be used, it to detonate the enemy mines because, it is better, and faster, to disarm the armed enemies in person.