I sometimes put a "Select" button in the portal row that places the ID of the portal row in a field on the main table. This can then be related to another Table occurrence of the portal table to display fields in another tab. Be it in another portal or simply on the layout.
I have a video of this on my youtube channel: Click here to see it.
The "active portal row" means that this specific layout object has the "focus" due to user actions such as clicking something in the portal row or tabbing into a field in the portal row. The same type of user actions (or scripted equivalents) also change the focus to something else. If you need to keep track of what portal row had the focus, you can use a script to save that active portal row number in a global variable and then use go to portal row to restore the focus.
#To save the portal row number:
Set Variable [$$Row ; value: Get ( ActivePortalRowNumber ) ]
#To return the focus to the portal row:
Go to object ["put object name of portal here"]
Go to Portal Row [$$Row]