Well Im not any sort of expert but I think they call the difference playing horizontally /melodically, for chordal improvisation, and vertically for modal /scales, So say you are playing an instrument where you could set the notes for example a sitar, you could play melodically around the melody lines and chords involved, or as one does in Klezmer, pick scales that dont correspond to our western scale , for instance one with an F# and a Bb, and also then play around the modalities. Ionian phrygian dorian myxolydian aeolian lochrian, and any others Ive forgotten. I must admit I am intrigued by the various modalities and Klezmer type scales but although I recognise Miles' cleverness that type of improvisation leaves me generally cold.