150 Enhancements have been added to the game, each unit has 3 Enhancements that you can unlock by killing enemy Elite units. To unlock the tier 1, you need 5 XP. You gain 1 XP for killing an Elite 1, 2 XP for Elite 2 and 3 XP for Elite 3. You must unlock the tier 1 before you can unlock tier 2, and likewise for tier 3. Tier 2 requires 10 XP, and Tier 3 requires 15 XP. TL;DR, if you kill 10 Elite 3 (30 XP) Mahzoon's for example, you'll unlock all the Enhancements for her.

By default, you'll have one Tier 1/2/3 slot unlocked, each tier has 3 additional slots you can unlock by completing different achievements.

Note: The next day or two will contain a patch that brings much more quality of life to this new feature, such as seeing your enhancements in the middle of a run, enhanced units being indicated by an icon, and more filters for searching enhancements while creating your loadout.