To limit a report to just certain records, you have many different ways to manipulate the found set to get the records you want. The simplest is to just perform a find for the records you want.
Would that be before or after the report has been made?
That depends on what you mean by that and on the design of your database.
- Create the layout with the needed objects for your report.
- Peform a find on this layout to pull up the desired records
- Sort your records if desirable
- Preview, Print, and / or Make a PDF of your report
Once you've created the layout, you can perform any number of different finds and sorts to produce reports based on the same layout but with different information.
In reality, to get the desired group of records for your report, you can perform your find on any layout that refers to the same table occurrence (box in the relationships graph) in the "Show Records From" box located in Layout Setup...