Q: When should I use exterior type for a surface?

A: You should use exterior type for a surface that normally belongs to the interior of a room but you want to use it as exterior, i.e. providing less scattering at lower frequencies. When a room is loaded into ODEON, the surfcases are automatically distinguished between exterior and interior ones, according to the Interior margin setting in the Room Setup. By default, the interior margin is 10 cm from the boundary surfaces in the room. This means that surfaces falling inside the interior margin are considered as boundary surfaces, displayed in black, while surfaces deeper inside the room and away from the interior margin are considered interior surfaces. These are displayed by a greenish color. An interior surface provides quite high scatering throughout the whole frequency range, while an exterior surface is expected to provide less scattering at low frequencies.

You can force a surface to be exterior, even though it was detected as interior, if you believe that due to its shape the scattering is less at low frequencies.