bandwitdth

I am confused by wirelssmode and bandwitdth settings. I try to goole it ,but few article about them. I hope to get your help here ! Followings are my questions: 1. Why a/n/ac mixmode can set 3 optional bandwith: 20MHz, 20/40MHz, 80MHz? The 80MHz is only appled to ac mode? Aren't they...