The error core page is used to display informative messaging to members when they attempt to interact with pages they don't have access to.
For example, if they attempt to access a page they do not have access to based on their access rights, if they attempt to log in after their account has been canceled or has expired, etc. There is only one error core page on your site which is used by all members regardless of their access rights.
You can put whatever content you want on the error core page. To include a MemberMouse-generated error message, use the MM_Error_Message SmartTag.