The Slabs that you unlock by defeating the Visionaries in Deathloop are your key to supernatural powers. When you equip them, they’re going to augment your abilities, giving you the chance to teleport around the map, increase your strength, and link targets together, and damage them all at once. As a result, they’re beneficial, and Slabs can become even more powerful by upgrading them.
If you want to upgrade any of the Slabs during your loop, you need to complete the Ubiquity mission. By completing it, you’ll unlock Residuum, the resource that allows you to bring certain guns, trinkets, and Slabs with you whenever you die in a loop. When you have this, you then need to acquire a Slab and then infuse it. Infusing an item allows you to bring it over with you using the Residuum you’ve been collecting.
When a Slab has been infused to you, all you have to do now is collect that Slab again during a different loop. It should be available on one of the eight Visionaries, or it could be on Julianna when she invades you. Regardless of how you get it, when you have a Slab infused on you and collect it again, it upgrades and becomes more advanced than it previously was.
You’ll have to collect a Slab at least two more times during different loops to upgrade it to its highest level successfully.