stereo sound has twice as much information as mono. Yet, the information
on one channel of stereo is often identical to the
other. For example, a bass guitar that 'appears' centered will have
the same data on both left and right stereo channels.
Joint Stereo carries identical information (like the bass guitar above)
in one channel, and the other channel carries the difference (anything
left over). This allows for a more efficient allocation of bits; however,
in practice joint stereo is only used with low bitrate signals. In
other words, this step may not be included, and whether it is included
or not depends on the encoder.