Non-character classes use levels, similar to characters, but are not meant for player characters. For example, the Militia class is the same as a Fighter, but flat out weaker. You would never be a Militia class if you had the option to be a Fighter.
Non-Character classes use the same math as character classes, but have a slightly different advancement. They do not gain XP, instead a GM simply sets the level. Some classes have level limits. For example, the Militia Class cannot go above level 5. These characters should have been Fighters in the first place.
Non-character classes are great for designing certain creatures and challenges, and also help understand how the world functions, specifically with how some artisans can get so good at a craft without leveling in a “combat class”.