Maybe one in a thousand sites will need to bind a definition to an interface. The 99.9% of the time it's not necessary, it's likely to cause mysterious problems.
I would say that you should never choose Internal, External, DMZ, etc.; always choose > unless told to change it by an Astaro Support engineer.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005