What does it mean to have fast or autonomous switching “enabled” and “disabled” on the same interface?

You can use same-interface fast or autonomous switching in cases where you have Frame Relay or Asynchronous Transfer Mode (ATM) WAN links configured as subinterfaces on the same main interface. Another situation is when you are using secondary networks on LAN interfaces, as during IP address migration. In order to enable same-interface fast switching, use the ip route cache flow configuration command.

If you enable same-interface fast or autonomous switching, packets whose source and destination address are the same are fast or autonomous switched.