The exact conditions may be slightly different in owner / service manuals for different cars.
For example, the 96-98 Civic service manual describes the following as severe conditions:
-driving for less than 8 km in normal weather, or 16 km in below freezing weather
-extensive idling and long periods of stop and go traffic
-driving in extremely hot conditions (32-35*C +)
-trailer towing, driving in mountaineous conditions
-driving on muddy, dusty, or de-iced roads
You should follow the severe if you drive mainly in the above described conditions. That means that you should follow the normal schedule if you only occasionally drive under these conditions.
A UOA will help you determine the ideal OCI for your car; it can even be more than the one described in the manual.
