Originally Posted by: tbrockWhen you speak of a scale off the CAGED system ...[/quote]
I discuss how to understand & use the CAGED system here:

http://www.guitartricks.com/tutorial.php?input=383

The trick to using CAGED is to realize that it consists of movable shapes that can be any chord depending upon where you put the root note. Look at the images on the bottom of the lesson pages in my tutorial. You will see the circled numbers, which are the chord tones (1-3-5) derived from the major scale.

Place the circled number 1 (the root note) on a note, and you have that chord. You have the right idea in your example:
[QUOTE=tbrock]For example, using the A form, perform a barre chord at the 5th string, 8th fret, would we be utilizing the F scale?

Precisely! Yes, you are making sense. And if you move that "A form" up two frets to the 10th fret, you are playing a G chord; if you move it down two frets to the 6th fret, you are playing an E-flat chord.

Hope this helps.