Expertise represents Skill in a broad area, such as an entire profession or role. It has a score.
It is used primarily to derive Skill scores and limit available Features and rule options.
In Futile Kingdoms there are only 4 Major Expertises representing the 4 classical roles of fantasy dungeoneering:
They have a score equal to the character's Level, or half rounded down, see below.
As adventurers gain Levels, they become larger-than-life heroes and eventually legends. The rules are designed to reflect that, while keeping things more realistic on the lower end.
Not every character has a Major Expertise -- in particular, NPCs unsuited for adventuring usually don't. You can leave Major Expertise blank and gain high levels along with corresponding Features regardless, although Features requiring a score in a Major Expertise remain out of reach.
Major Expertises can be Primary or Secondary.
Any character who doesn't have a Primary Expertise yet can pick one during Downtime.
Primary Expertise is always equal to Level.
A Secondary Expertise must be bought via the Second Expertise Feature. It is treated like any other Major Expertise,
but its score is only equal to Level//2.
No character can have a third Major Expertise, although they can theoretically have an unlimited number of Minor Expertises.
The famous weaponsmith might not be a great hero, but he does know his craft. The GM decides that he has the (informal) Weaponsmith Minor Expertise at +4.
Minor Expertise reflects areas that are important enough to be covered by the rules, but not enough to justify an entire role for an adventurer. It is not necessarily tied to Level, with a score either determined by the rules or written down.
Formalized Minor Expertises include: