If you don't want to set these parameters on the primary database during initial configuration then After a switchover scenario you need to set them for reverse log shipping.
I have some systems that have set FAL_CLIENT and others that do not. FAL_SERVER is the most important one because if a log is missing, this tells the standby where to go get it.
And you don't have to have any of these set. They are only important for missing log files.
fal_server = <physicalStandby>
fal_client = <Primarydatabase>
And then switch them after a switchover for consistency and transparency.
Wait...the FAL_SERVER parameter points to the database where FAL will obtain a missing archived redo log. On the primary, FAL_SERVER points to the standby. On the standby, FAL_SERVER points to the primary. Since the primary and the standby maintain their own parameter files, there is no need to switch them.