3.5 Primary Category
If your post(s) or WooCommerce product(s) have more than one category enabled, then you can set one of those as a primary category.
In the post editor, selecting more than one category or subcategory will display the Primary Category drop-down. Select the category/subcategory you want to set as the primary category using the drop-down.
Similarly for WooCommerce products, selecting more than one category or subcategory will display a Make Primary link next to each category and subcategory. Click the link of any category/subcategory to set it as the primary category.
You can disable the primary category feature, by adding the following code to the wp-config.php file
define( ‘SMARTCRAWL_DISABLE_PRIMARY_TERMS’, true );
Primary category in post breadcrumbs
If the breadcrumbs feature is enabled, the chosen primary category will be displayed as the category/subcategory in the posts’ breadcrumbs. And, the chosen primary category will be displayed in the post or product URL if the post or product permalink structure has a %%category%% or product-category tag.