According to it

Part-time 4WD
This type of system is best for people who really don't need 4WD, but occasionally go off-roading and want a little extra security when it snows.
Full-time 4WD
is best for people who regularly drive in slippery conditions where constant shifting between 2WD and 4WD can become tedious
All-Wheel Drive
 work well in cars and crossover vehicles because they don't add much weight and make them true all-weather vehicles

If you want off road:  Select the system allows the driver to switch 4WD

http://consumerguideauto.howstuffworks.com/your-ride-4wd-vs-awd-cga.htm