You have to be able to define 'new information'. You can get FileMaker to 'mark' the records which have new info by something which monitors the entries. You can search for modification dates > dd/mm/yy or you could have a checkbox which enters 1 when a record is changed and which is emptied when it has been printed. Before printing you would just find all those >0. There are many other ways to do this...
I do not understand exactly what you are saying about the postcodes... but also I do not understand why you would have postcodes in a checkbox.
we have 300 areas that we support with persons. Now we must contact all customers where we have a support-persons (based on postcodes).
In my customer list i have also customers where we have currently no supportperson.
The supportperson handles more then one postcode. Currenlty i define this using ceckboxes. (support1 handles Checkbox 100 and 200)
In this case all customers in postcode 100 and 200 must receive a letter with the contact details of this person.
Currently when i switch to my print layout, i also see the customers that have no supportpersons.
I only want see in my print-template the complete letter (not including the customers without a supportperson)
Hope this is now better to understand.