The Major sixth voicings contain the root, the major third, and the major sixth. Major sixth chords and Major seventh chords are generally interchangeable in usage.
Please note that these voicings are actually inverted minor triads. (C6 is the same as Am pitchwise, it is the way the chord isused which differentiates it's function and sound)
as always, you can use a 6th string or 5th string root.
The lowest note in these voicings is as usual the root,and it tells you what chord you are playing.