Briefly, Layer III can use any of the following stereo encodings:
regular stereo L and R channels are encoded independently
middle/side (M/S) joint stereo L+R and L-R are encoded in place of L and R
intensity joint stereo high frequencies are encoded mono with some stereo imaging info
M/S + intensity joint stereo a combination of M/S for low freqs and intensity for high freqs
when the player says "joint stereo" which one is it?