Happy to hear that you're working on the Soloing Concepts lesson!
Yes, I can see why this might be a bit confusing.
Maybe you already know how the modes work, but just in case you're not familiar with this subject;
The modes within the major scale are these:
Ionian (that's the major scale)
In other words, if you're playing in the key of C major for instance, the notes are:
C D E F G A B and back to C
Playing these notes from C to C creates a major scale (Ionian mode).
, but if you play the same notes, starting on the note D.
D E F G A B C ,.. you get the D dorian mode (which is a minor "scale".
E to E within the C major scale would be the phrygian mode etc etc.. so all these modes contains the same notes.
C Ionian mode (same as C major scale):
C D E F G A B
D E F G A B C
E F G A B C D
F G A B C D E
G A B C D E F
A B C D E F G
B C D E F G A
Now to your question...
When you look at those diagrams that you're referring to, you can see that one of the tones are white, and that note represents the root of that mixolydian mode. In other words, if you start on that note, you'll hear that it's the correct mode.. the A mixolydian mode. I've just notated the mode to the lowest note in that particular position of the guitar, so that you can learn the mode all the way from the lowest string to the highest string. If you're starting on the lowest string (like it shows in the TAB/notation) it will create the sound of the different modes depending on which diagram you're playing. I should've made a circe around the root note both in the TAB and notation that indicates that that note is the root. I shall modify this TAB to make this more clear.
Thanks so much for pointing this out.
So just make sure you start and end on an A note when you're playing those positions of the A mixolydian mode.
Let me know if you have any questions or if this message was a bit too confusing.