Stereo panning using the Stereo Effect
If you pan a stereo track hard left or right using a track's Pan knob you'll only hear one channel of the source signal. The other one is muted. It's quite normal for a panner to behave like this, but it's not always what you want.
Consider a stereo piano sampler patch (or a piano VSTi). Sometimes they have a very wide stereo image, where the bass keys are on one channel and the treble keys on the other. Panning the piano using the track's Pan knob will change the sound of the piano considerably. Furthermore the stereo image may be too wide for your song.
You can use the Stereo Effect to pan a stereo signal in a different way. Just leave the track's Pan knob at center position, and load a Stereo Effect in the track's last (right-hand side) effect slot.
Now open the Stereo Effect. There's a Pan knob for each channel. If you want to pan the track a bit to the left you can keep the left channel's Pan knob at 0%. Turn the right channel's Pan knob down until you reach the position you're after. Now both source channels still appear equally loud in the mix. Note that the source's right channel would have been softer if you used the track's Pan knob.
You can also use the Stereo Effect to reduce stereo width. Example: Left channel Pan = 25%, right channel Pan = 75%. Now the track is still panned center, but it's not as wide.
Giel Bremmers