From the original guide:
But you actually can’t attack every planet you want to. So players smaller then 35% of your networth are protected. In war, this limit is reduced to 25% and if you have been attacked by this empire before, the limit is removed completely. Also the home planets of other empires are protected, except its an inactive in your family. When an empire is in vacation or blocked or lost 50% of its planets without attacking back, then their planets are protected as well. When having no morale left, you can still send out your fleets, but you’ll have to wait until your Morale is positive again, until you can trigger this attack.
A few things:
This isn’t intended as a long term fix for anything. Rather, it is restoring a feature that was removed in 2015, brought back, and disabled again due to unanticipated exploits.
The purpose here is to restore the pre-2015 balance on attacking. It will not solve everything, but it will hopefully improve things somewhat while we work on more long term changes that will eventually render it unnecessary.
This will come back as a galaxy option, with the default being to have it enabled.
Family-based limits are not part of this change, as they were not part of the original feature. There is an existing thread about the feature to Restrict attacks on smaller families for those who wish to discuss it.