In all the tables, there is a field called "one". This a calculation field defined to return a value of "one" (1) What is this field for, and how is it used?
That was a way to create a cartesian self relationship in version before 7.
But sometimes is still used.
You can also use the "one" field as a match field to calculations in other tables. Your calculations would return a 1 for true and 0 for false. For example you have a table called customer and want to have a portal list showing only their open invoices. In the invoices table you could create a calc field called status calc with the following calculation
case (invoices::status= "open" ; 1 ; 0)
Then create a link from customer:: one to invoices::status calc
This way the portal to that table will only show the open invoices for that customer.
Hope this helps,