Exterior support is only on the outside "visible" part of the object.
 Anything past the "visible perimeter" is "interior" or "Full Support"

So an object I made with tinkercad:  
https://tinkercad.com/things/bMy319N45M0-daisy-powerline-15xt-bb-loader

The inside of the bowl requires support so I picked "Full Support", the
portion that supports the upper portion does not require any support as
made those angles greater than 55 degrees.  The default in Skeinforge from
MBI is 45 degrees (Raft Tab -> Support Minimum Angle).  I made a test
object and saw no drooping with the plastic at 55 degrees so I changed that
setting.

Another example would be if you had a mini bar stool with a person sitting
on it.  If you selected Exterior support it would build support for the
person's arms, but not the bottom of the seat.  You would have to select
"Full Support" for it to build support material for the seat bottom.  Now
if the machine could bridge the gap "open air or bridging", support may not
be required.

In case your not aware if you want to print support material but not have a
raft change the following on the Raft Tab to zero (0), you still select
"Raft/Support" but will not get a raft:
Base Layers 1 (default value)
Interface layers 2 (default value)

Okay that was much longer then I had planned :)

Ben