You can use “user name” when referring to a real name or full name. If you are referring to an identity used with a password, you should not be used the space between the user and name and use “username.” So far as the “user-name” is concerned, it is rarely used.
Why should not be used one username for Everything?
There are some online services and websites that use your email address as your username; some services don’t do like this. This approach either works, but there are various services that do not use an email address as a username and need users to create a new username with a combination of letters and numbers. The next text will be your username that will be used to log in to that service when you use it. Sometimes the service may apply a condition for a minimum number of characters and sometimes limits the number of characters for username. Some of the services will not allow you to use your email as a username, while some allow you to use your email address and a password to log on. Why is it so? The reason for that is discussed below?
A reason is that some services use an existing number or member ID to identify you on the network. The IDs or numbers like credit card number, student ID, Social Security number, auto-club membership, bank account number, and health care member ID are already used to identify someone’s account information. Consequently, an online service sometimes may ask you to use that IDs or numbers for your username. Although most time, services need you to create a new username with a combination of characters or numbers, and that’s different from your student ID, Social Security number, auto-club membership, bank account number because they want to keep your username more secure.