Merchant Shops such as Google Shopping and Facebook Dynamic Ads have their own set categories that you need to follow. Hence, you need the category mapping option. You can map the products of your categories to be included under the category of the Merchant shop.
Here, you need to name the custom mapping, choose the category you wish to align with your Merchant Shop category and provide the Merchant Shop category to be aligned with.
Categories are the primary key to get products into the search result. So, the practice is to assign the most matching shopping engine category to your store category for each product.
Once you set a Category Map, you can make it effective when creating a new feed by choosing the Category Map you created under Value for an attribute.
If you scroll down, you will find the Category Map section under which you can choose the category maps you set.