Primary category selection and Product Detail page breadcrumbs
Key Takeaway
The primary category builds the breadcrumb on the Product Detail page. If that category isn't included in navigation ("Show in Menu" isn't selected), the breadcrumb shows Home instead.
In a B2B Commerce store, a product can belong to several categories, but only one of them is its primary category. The store builds the breadcrumb on the Product Detail page from the primary category, so your choice of primary category controls the path buyers see above the product.
Categories and the primary category
A product can be assigned to more than one category, so buyers can find the product through several navigation paths. Even so, a product has only one primary category. The store uses the primary category to build the category path in the breadcrumb on the Product Detail page.