Stuart, assuming I understand your question, to understand what the Power On/Power Off selection in an activity does you need to go back to how a device is set up.
Within the device settings there is a toggle to ‘Always Turn This On’. With this enabled in a specific devices settings, any activities created in the room the device is linked to, with Automatic Commands turned on and Activity Type set as Power On, will automatically include a command to power on that device. Conversely any activity with Automatic Commands turned on and Activity Type set as Power Off will automatically add a command to power off that device. Essentially it saves the effort of having to create some of the commands in your activities.
With regards to the power button in v5 (either the ‘Power Off Activity’ option in the drop down at the top of an activity panel or the power button on the chosen room screen), unless I am missing something, there is currently no way to configure it, well not like you could configure a power off activity in previous versions of the app. Using either of these two options will simply power off any devices linked to that room that have the toggle switched on for ‘Always Turn This On’ within the devices settings.
Apologies if I’ve mis-understood and you already know this.