Primary category.In plain English.

Google says the categories you select affect your local ranking. You choose one primary category from Google's list, and you can't create your own.

Pick the most specific category that fits. Google's example is 'Nail salon' instead of 'Salon'. Additional categories can cover other parts of the business, but Google's guidelines say to choose the fewest categories it takes to describe the core business, and not to use categories solely as keywords.

Google may also use your categories to label your business, and adding or changing a category can mean verifying the business again.

Example: a business that is mainly a plumbing company chooses Plumber as its primary category, even if it also sells some appliances, and adds an additional category only for a real, separate part of the business.