Ok, quick question (possibly) what does it mean if somebody says a chord is 'Lydian based' or 'Dorian based' etc?
Lydian and Dorian are modes or certain scales.

So if a chord is Lydian based or Dorian based I think it means that it uses the same tones as the modes of the scale in that key.

if that makes sense
^Yeah, more or less. A mode based chord is a chord derived from a certain mode.

Take Lydian: 1 - 2 - 3 - #4 - 5 - 6 - 7. What seperates that mode from the major scale is the raised fourth, (In F, Bb up to B) - so that's the tone that's going to emphasize the sound of the mode. The same works with chords.

Build a maj11 chord in F Major, and you end up with: F - A - C - E - (G) - Bb which would be Fmaj11.

Then build the same chord in F Lydian, and you end up with: F - A - C - E - (G) - B which is Fmaj7#11.

Because of #11 in that chord, it's associated with the lydian mode, and ergo based on the lydian mode. (The maj7#11 is sometimes referred to as the Lydian chord anyway).
Oh i think i get it, so a maj7#11 chord can only occour as the tonic chord in a lydian mode

Thanks for the help JohnlJones.