The Root Category in the category tree is like the doyen in a family with many offsprings. You will see that the category tree has the upside-down structure because the root is on the top with many subcategories in the lower level. And each category contains many products in it. You can see the following category tree:
In the example, Default Category is the root category and it has many subcategories such as Women, Men, Gear, Training.
To create new category in Magento 2, allowing store admin to create as many root categories with different structures, different stores and different views as needed but there is only one root category that is applied for your store at a time. Moreover, customer can not view the root category and there is no URL Key supported for it
How to create a new Root Category in Magento 2 as the following steps.
Product > Inventory > Categories.
Add Root Categorybutton.
General Informationtab, you need:
Is Activefield to activate the root.
URL Keyfor the new root.
Descriptionabout the root category (Optional).
Choose Fileto upload an image.
Meta Keywords, and
Meta Descriptionof the new root for the better SEO.
Include in Navigation Menufield.
General Information, move to
Display Settingstab and choose Yes for the
Save Categoryto use the new root category.
Save Categoryto complete.
Stores > Settings > All Stores.
Stores, hit the
Main Website Storelink.
Root Category, choose the new root category.
Save Storeon the top right corner to apply the category.
Ref: Magento 2 User Guide
It comes to the end of tutorial: How to Create a New Root Category in Magento 2.