An 'undefined' page is any page that has been linked to from within Confluence, but that does not yet exist.

Undefined Page Linked From
CC Page: Error: "The bill's document style is not valid" when previewing a bill (Practice Solutions: Support Notes)
Error ‘Invalid Merge Field’ when performing a mail merge which contain extra fields or lists Page: Mail merge fields not merging the correct information for Extra Fields or Extra List items (Practice Solutions: Support Notes)
PR Page: Client name not appearing on fee invoices or debtor statements (Practice Solutions: Support Notes)