Documentation on how to implement Custom Territory Dimension: Using Extension Dimension

Hello, Everyone.

I would like to seek for your Expertise regarding Territory Setup for CX Cloud. I have some experience on configuring Territory Dimensions especially for Custom Dimension using Auxiliaries. However, all three Auxiliary Dimensions has been used in our setup. I noticed these new available options in Enabled Dimensions and Metrics Select and Add: Dimensions.

Dimension Name: Extension {Number} :

with a Description: "An Extension dimension created from a standard lookup type. A lookup type must be selected which provides the dimension member values."

I'm trying to look at specific documentation on how to implement Custom Territory Dimension: Using Extension Dimension. Hope someone could help.