Assuming that you have this data model:
And you want to pull up all Invoice Data records for the current found set of Invoices and then constrain the found set to only invoice data records for a specific state, your script should be:
Set Variable [$$TAX_REGION ; value:Company Dashboard::Tax Region Sort]
Go to Related Record [Show only related records, Match found set; From table: "Invoice Data; Using layout "Monthly Sales Tax" (Invoice Data); new window]
Enter Find Mode  ----> clear the pause check box
Set Field [Invoices::Tax Region; $$TAX_REGION]
Constrain Found Set [ ]
But since you are using Go To Related Records with the "match found set" option, you can also first perform a find on your invoices layout to limit your original found set of invoices to just one state and then the constrain will not be needed, just the Go to Related records step.
And it is also possible to produce a report where the line items (invoice data) records are grouped by state rather than limiting the records to just one state. Each group can start a new page also. This can be done by adding a sub summary layout part (or two parts, above and below the body) that use Inovices::tax Region as their "when sorted by" field. You would then use GTRR (go to related records) followed by sort records to sort (group) them by tax region.
It's also possible to use a sub summary layout part in place of the layout body part (When sorted by product ID) to produce a report with one row for each product sold in each tax region.
For more on summary reports, see: Creating Filemaker Pro summary reports--Tutorial
Thank you Phil, worked like a charm.