Need help creating a sub-category in a document, solved by making a reference in the category.

Hey everyone! 👋
I am trying to make a sub-category in a document but I cant figure it out. Tried searching Google and this Slack channel but I feel like I might be searching for the wrong terms?

I mocked up an image of what I want to get basically:

Any help would be great thanks!

Just a follow up to this, I ended up making a reference in the Category which referenced the Continent. This works fine :)