What is a Custom Region?

https://vuulr.com/settings/regions

Group countries into your own custom region to match your Listing's availability. Custom regions can be easily edited and updated at any time, giving you the flexibility to define exactly where your Listing is available.

NB - You can do similar with Languages - if you need a default grouping for a Listing's language options, you can create a Custom Language Group https://vuulr.com/settings/languages