(From OneDND, this is a grouping for classes, supplying an ability that is tied to the group and thus is given to all classes in the group. This is not homebrew made by me, it is a work-in-progress of WotC.)
As the name implies, these adventurers are those who have knowledge in specific and specialized fields whether that be in dealing damage, tracking, magic, or whatever else takes their attention.
Expertise:
At a level, depending on the individual class, choose two of your skill proficiencies or tool proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.
At a higher level, depending on the individual class, you can choose two more of your proficiencies (in skills or tools) to gain this benefit.
Classes:
Rogue
1 & 6
Ranger
2 & 9
Bard
2 & 9
Artificer
2 & 9