Oddity XP System
The Oddity XP System is an alternative experience system which rewards exploration, introduced in upcoming version.
The main purpose of this system is to allow players to level at a similar rate regardless of what their build and play-style is. Going through the world stealthily, guns blazing (or in some cases diplomatically) or anything in between should now earn you similar amount of experience.
Oddity items
These items are scattered throughout Underrail, mostly at points of interest. Some of them are gained by killing critters and other specific types of enemies, but since each oddity item can be studied a limited number of times, you can only farm XP from a single creature type to a certain point.
Also, there are generally more instances of an oddity item type placed than you need, so you don't have to have 100% exploration score to get them all. Though, some of them will be unique and only awarded for discovering special areas or defeating bosses.
They cannot be found when playing with Classic XP System.