When we announced Trello, we did not support wildcard subdomains.
Let’s say your app needs to be able to authenticate Trello users across a wide variety of subdomains. You would register https://*.my-app.com which would allow https://an.example.my-app.com and https://other.my-app.com. Note if you also want to allow the root https://my-app.com you will need to register that separately.