Magento has a setting called "Anchor" at the category level, which can be found under admin Catalog Categories Select a category Display Settings Anchor.
Magento displays all products in the current category and its child (sub) categories if the "Anchor" setting is set to Yes.
As an example, suppose the category "Mens" has zero products and its sub-category "T-shirts" has some products, and the "Anchor" setting for "Mens" is set to "Yes", all products from "T-shirts" will appear in "Mens".
By Default, Klevu does not follow this behavior when it comes to Category Merchandising. Klevu will only index the categories which are assigned to a specific product. So if in the above case, the “Mens” category has no products assigned, the “Mens” category can not be used for merchandising purposes.
To handle this, the Klevu module has a setting called Respect is Anchor Category showing under Stores Configuration Klevu Tab Search Configuration Product Attribute Settings. Please set this to “Yes“ to enable it. Once you enable it, please contact Klevu support to confirm that it is working.
By default, this setting will be selected as No.