Power for all activities mean that if you have that option on, the device will turn on automatically on every activity that you have on your setup unless you manually turn off the “automatic commands” option under each activity.
Power device (in an activity) means, if it is on the device will only turn on and off for that particular activity like a blu ray player that needs to be on only when the watch blu ray activity is on, unless again like the above, you turn off the “automatic commands” option manually and delete that command from your start and stop command. I hope this helps.