Custom Messages are displayed to a user when the user encounters a specific type of error.
It applies to all the error messages that fall in the categories below:
User creation disabled message: When a user with certain roles is restricted from logging in. A customised message that is added in this category will be displayed to the user.
Restricted Domain error message: When a user with specific domain is restricted from logging in. A customised message that is added in this category will be displayed to the user.