Techmaster Pocket is a gear perk that raises how many proximity gadgets and C4 you can carry. It is the core of a trap build, giving you more charges to mine roads and outposts.
You hold more proximity gadgets and C4 at once. You can set up more traps before you need to restock.
Wear the Techmaster Pocket piece of the Techmaster set. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
The rest of the Techmaster set: the Pants for damage, the Boots for Proximity Prevention and the Cap for Tech Scout.