How Do Environment Variables Work?
The custom label value in UAT that will replace the value of Dev1.
The value in UAT should be replaced with https://UATfakeurl.html.
Commit the Account Summary custom label in a user story.
When promoting the user story, the {!Environment.ACCOUNT_SUMMARY} key in the feature branch is merged into the promotion branch, and it will be replaced with the value https://UATfakeurl.html when deploying to UAT.
This is the result after deploying:
In the Deployment record, you can see that there is a replacement.