I'm not sure that I understand your question. But FileMaker is very "focus sensitive" in many areas. If you click on or into a portal row, this row gets the focus and many references to the portal's data via its table occurrences will now be from the context of that selected portal row.
This is often exploited in FileMaker solutions by setting up a portal as an improved substitute for the standard drop down list or pop up menu. You put a button in the portal row (which can cover the entire row), and the script can transfer data from a field in the portal record into a variable or field as a way to select data from the list in the portal.