A business category slug; the unit of directory grouping and recommendation. Maps from Google place types.
A Niche is a business-category slug (for example bakeries, plumbers, restaurants) — the unit Doozer groups, recommends, and builds directories around. niche_mappings maps Google place google_type values to a canonical niche_slug and a master category.
Gotcha: businesses.primary_niche is inconsistent (older rows hold a slug, newer rows a Google type). The helpers in includes/nearbymenow-data.php (nbmnCanonicalNicheSlug, nbmnNicheMatchValues, nbmnNicheInClause) normalize both. Recommendations link to the business's OWN canonical niche (so it is always listed), not the master category. Niches are nodes in the OKF network graph and roll up to categories.