One of the core items to get right when looking to optimize each of your location's GBP— and to achieve the highest possible map placement and total traffic — is to set the correct category assignments for each location.
Primary Category
This is regarded as one of the core decisions for each location and is therefore also rather complicated. A primary category should be a descriptor of your business as a whole, as it's the top factor that goes into ranking, aside from your physical location. Without a fitting primary category, you cannot rank for your most valuable searches.
Ultimately, when choosing your primary category, it is better to be as specific as possible. For instance, if you own a chain of pizza restaurants that offer delivery and takeout, your primary category should either be Pizza Delivery or Pizza Takeout (as opposed to the less specific Delivery Restaurant or Takeout Restaurant).
Additional Categories
Google currently has over 3,900 different categories, and while there is some industry controversy around how many additional categories a GBP page should have, there doesn't seem to be any drawback from having more than a few.
Additional categories should be complementary to the primary, and it's also important to keep additional categories as relevant as possible. In the example before, if the restaurant was an Italian pizza joint that also offered table dining, they could add Italian restaurant as an additional category. If they truly specialized in a region they could add Northern Italian restaurant or Southern Italian restaurant. The general rule for additional categories is, "describe, don't spam or be redundant".
Make sure that these secondary categories don't stray too far away from main selection. The driving purpose behind adding this content is to enhance it and make it useful for customers.
Although deciding on categories is just one aspect of the entire task, ensure that you're handling categorization with thought and care.
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