With the release of Business Central 2020 Wave 1 lot of new features has been added to make product more prominent.
Though this concept of ENUM was introduced earlier but from new version onwards most of the option fields are available as extensible enum such as follows

If define enum is extensible then we can add more option values in that field.

How to add the new option value in existing enum field.
First create extension for enum as below

To check the value just added one action button as below

To check the output publish the app and check

Select the document type and check the result as follows

Hope this help you…
Stay tuned for more..