If you registered with Digidentity before the 3rd November 2017, you will have created an account using a username, made up of at least 6 characters (letters, numbers, dashes).
If you cannot remember your username, you will need to recover this part of your login by selecting "I have forgotten my username" on the Digidentity login page. Once you regain access to your account, your username will be "migrated" over to your registered email address. This means that you need to provide your email address and password when logging into your account in future.
If you registered after the 3rd November, your username will be the email address you used to register.