You can also select a subsection of the song, then u fire up the scale tool, and it will tell you what scale is suitable for that segment of the song. It helps alittle if u are trying to solo over a particular section and are clueless abt the "right" scale to use.
As far as i know, how it achieves this is by scanning the song section for all the notes used in that section, and then cross checks with all the available scales in the GP5 database.
That is also why the scale tool tends to reveal too much sometimes, because to GP5, C major is equivalent to D dorian and E phrygian and so on.