Returns the organization's effective subcategory set: every custom subcategory the org has created, unioned with every global (canonical) subcategory. Each row is tagged type: custom | canonical and carries isExcluded, isShadowed, and the shownInProductDropdown rollup. With no parameters only rows where shownInProductDropdown is true are returned, which is the set a product picker should offer. Use id + type from this response when assigning a subcategory on product create/update: customSubCategoryId for custom rows, productSubCategoryId for canonical rows.