When setting up a Coviu account the plans will ask you for your "Practice Name". Depending on what Practice Name you choose, it will create a subdomain for you in the next line - this is called the "Practice Link" and is composed like this: https://<yoursubdomain>.coviu.com . This link provides a unique link for your practice and the chosen subdomain cannot be used more than once.
If you have multiple Coviu accounts and would like to re-use a previously used subdomain so you can use it for a new account, you need to do the following:
Login to you Coviu Account
Click on Configure -> Account details
Under the unique domain field - change this to something else to release your domain. Then click save.
Your unique subdomain has now been released and you can sign up to a new Coviu account with your unique subdomain name.