The Rakuten Category Taxonomy and category list is on a per category basis.
Rakuten categories can be quite intimidating with thousands of categories. The primary category is required as is defined by you. Make sure that top-level categories are used only instead of using bottom-level categories.
Using both top and bottom-level categories as the primary category can be problematic as character limit of category field may be exceeded. Secondary categories are optional and should be delimited with double tildes.
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