We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
I'm very new to the game but I think when things refer to 'efficiency' it refers to the cost of doing something. Thus in this case, the cost of defending an area is less. Therefore you would lose less organization in defending.