Apple Logic Pro 9 (TDM Guide) - Manual (Page 12)

Depending on whether a mono or stereo region is played back, the Pan knob will
behave as either a Balance or Pan control. If you play back a mono region when the
Pan knob is centered, both channels of the audio channel strip will output the same
signal level.
Note: Universal Track Mode has limited routing capabilities, as it doesn’t allow separate
handling of the left and right audio channels. The Non-Universal Track Mode is useful
if you need to play different mono files for the left and right (mono) channels of a single
audio channel strip, even when it is assigned as a stereo channel. A stereo input and
stereo output plug-in inserted into this stereo channel receives different signals for the
left and right channels.
12 Chapter 1 TDM Audio Hardware and Drivers