About the relationship, I am not sure about it.

If one customer, have two invoice. If we want to see the two invoice in a form.
What should I create, to see just the two invoice instead of the other invoice?

For example I have a form for just viewing of invoices.
I want to find the invoices of this particular customer.
And the form only shows these two invoices when I find it. That is when I use a navigation toolbar to browse through, I wouldn't be able to go into other invoices.
Then, when I finished it, I can resume going back to the normal form which able me to view the whole table.

Hope I am not confussing you.