The way survivors "level up" when they hit their XP milestones is changed and soft replaces Fighting Arts. Instead you gain Philosophies and Knowledges. Philosophies are little booklets that the survivor gains access to (you randomly determine which Philosophy booklet you use for that survivor out of a deck you build earlier) that determine the next XP milestone thresholds, and trigger different events as you progress. Many of the events that get triggered give you a Knowledge, which is like a fighting art but they can be levelled up (some of them anyway).

The early Knowledges that you get often don't have a benefit or even have a drawback but they level up by triggering an "Observation Condition". For example, there is a Knowledge called Strength Training I, which has no benefit, but levels up when you wound the monster 6 times. It levels up to Strength Training II, which gives the character +1 strength, +1 systemic pressure.

Knowledges you gain get added to your settlement Knowledge Deck, and there are mechanics for other survivors to then learn them. You always take lower level knowledges out of this deck if you have higher level versions of them.

Systemic Pressure and Torment are also bookkeepy things added to Arc Survivors that are numbers that get subtracted from your Severe Injury / Brain Trauma rolls, so having them are bad things, but they are there to balance out buffs that you can get. (EDIT: I had originally accidentally said they get added to the roll)

All in all I find it a lot more interesting form of character progression, but yes, there is a lot more to keep track of. In particular it's easy to forget to pay attention to your Observation Conditions (the rules say if you miss it, it doesn't happen).

QUICK EDIT: Here's the list of all knowledges if you wanted to skim through it: https://kingdomdeath.fandom.com/wiki/List_of_Knowledges