Dominant 7th chords generated by the same diminished 7th chord are functionally related. You can use them as substitutes for one another or choose one as the basic voicing and combine it with any of the others to make an extended dominant.

Thinking of C as the tonic for both a major and a minor scale, here are major and minor 6th chords of that scale along with a diminished 7th chord. See examples 6 and 7.

Put the major 6th chord in Example 5 together with a diminished 7th chord, and you get what Barry likes to call the major 6 diminished scale.