Modern Musical Scales= Major, natural minor, harmonic minor, melodic minor. All of these scales have seven notes and each has its own individual "modes" (except natural minor because it is the same as the major mode) Major= dorian, mixo, lydian etc... but harmonic minor also has common modes like phrygian dominant, melodic minor also has commonly used modes like, the altered scale, and lydian dominant. So it is not true that all modes are diatonic or are from the major scale. Most of the latter scales are basically found only in modern jazz and classical, but nevertheless are still usefull. You could call the altered scale the seventh mode of the melodic minor scale. Modal music can be defined largely into two different catogories. First church gregorian chant modal music, which is a mid-evil form of music before westerners truly discovered harmony and harmony was the result of melodic strands sounded together, and no real chord progressions were produced.(but I'm sure you can find examples that coincidentaly form modern day chord progressions.) Bach and early Bruoque musicians wrote exactly the opposite, writing music with a defined harmonic content while connecting the Harmony with melodic movement, but the harmony was always in check. This music was also very polyphonic, and that was the big difference whith the classical period, which was about clarity and balance and had a much greater homophonic content. RULE #1 If music has no chord progression than it is definitely modal. 2nd type of modal music is modern Jazz, funk, Rock or rap or whatever, However, modal music can have chord progressions, but must coincide with the scale being used. an example is Kind of Blue by miles davis or Oye Comma Va by Sananta, both are dorian mode songs, but Oye Comma Va uses am-DMaj chord progression or a i-IV chord vamp, not to mention melodically you can use notes oustide the dorian mode and still be in a dorian mode jam, by playing (outside) I hope this helps, but I'm sure it will just add to the confusion. Remember no chord progression, always modal. Chord progressions using not diatonic tones, always non-modal. Chord progressions that only use one mode (especailly not Ionian, or Aoelian) usually considered modal, but it is up to the theoroists and how he analyzes the chords and the content of the tune. Modes are talk about by Chord scale theory, like the progression Dm-G7-Cmaj or a ii-V-I you could say the scales used over the Dminor chord use the    D dorian scale, over the Gseventh use the G mixolydian scale,or G altered Scale and over the CMaj use the C major or Ionian scale. trust me I do to Berklee College of music