For such topics you may want to lurk on ServerFault [1] and SuperUser [2] to get more detailed information. If you gave an example of your intended end goal it might help answer the question as this could be interpreted as a DNS, cloud or web server question.

The title of your post should probably be something along the lines of "Ask HN: Subdomain limitations and capabilities?". If you click edit you can change it.

[1] - https://serverfault.com/

[2] - https://superuser.com/