Parent products are categorized according to the category path. The category path specifies in which category a product belongs. This is only true for parent products not for child products since child products relate to parent products, not to categories. For child products this field must be left empty.
The category_path is a forward slash delimited string which begins with a top-level category and follows with sub-categories. When the product has to be assigned to more than one category, you can provide all categories, delimited by a | (pipe-symbol).
If a category path does not exist, it will be created and the category will be published.
Single category:
category/sub-category_1/sub_category_2
Multiple categories:
Category/Sub-category_1/Sub_category_2|Category2/Subcategory22|Category3/Subcategory33