u_username is for cases where you want to allow people to create their own username, instead of having their email address be the username, which is the default.
Also, u_username has a built-in feature where there system will automatically check for and prevent duplicates.
u_email can prevent duplicates as well, but that is off by default. You need to checkmark to enable it under Custom Fields/Notifications > Edit Email.
Keywords: API, username, user email
