UserMatchingModeEnum : * identifier - Use the source-specific identifier * email_link - Link to a user with identical email address. Can have security implications when a source doesn’t validate email addresses. * email_deny - Use the user’s email address, but deny enrollment when the email address already exists. * username_link - Link to a user with identical username. Can have security implications when a username is used with another source. * username_deny - Use the user’s username, but deny enrollment when the username already exists.