Q: Is there a difference between add2 and add9?
A: maybe but not really. add9 maybe implies that the 9 should be in the higher octave, but it's also fine to stuff the 9 into the same octave as the rest. the reason a person would choose to write "add2" on paper (and i do), is to avoid tricking the reader into thinking they should play the 9 way up high. and in that case, if i ever wrote 'add9', that means i'm probably saying i want the 9 to be high, because otherwise i would have written add2. however, for "maj9" for me it is usually assumed that the 9 is not high. just because, in the professional world 'maj9' has come to mean 'must include the 7 and 2 and 3 and optionally 5'