The regular arm rest is just a small flat part that covers the opening to the storage, maybe 1/2" thick. The extended one is longer and extends towards the front of the car and is 3 - 4" thick and has additional storage to put stuff. The bottom of it is the cover to the lower storage and the top has a hatch to it's own storage if that makes sence.

Some people claim it gets in the way of shifting (their elbow I guess) but I haven't had any problems with mine.

There is a pic here:

http://www.subaruparts.com/catalog/?section=148