for the User ID, you should select the primary key column of the users table.
the Username and Password settings allows you to select the columns in your users table that will be used for the Username and Password columns.
If you wish to use the email address as the username, then set the User name column to the email address column.
The Username and Password settings determine the columns that will be used for user lookup on the login form.
if the user record is found, the user is logged in by storing the User ID value in a session variable. The user ID is used to identify the user throughout the site.