Ford may have a slightly different definition of severe duty vs normal duty, but it appears AutoZone describes severe duty as:

"Frequently take trips under five miles
Make trips under 10 miles in freezing weather
Drive in extremely hot weather
Drive for long periods of time in stop-and-go traffic
Idle your car longer than normal
Drive in the mountains
Carry heavy loads with your vehicle
Drive on muddy, dusty or de-iced roads"

It sounds like it covers a wide variety of situations a car would normally encounter. Short of cruising for 50 miles at a time at 60 mph on flat/level highway roads, I don't think most cars would fall under "normal" duty.

 

Very true. Any maintenace schedules we run for customers is always following the severe duty list.