Just to add a little here:
Keyswitches are notes that are used to invoke different 'articulations' of an instrument, and are fairly common in orchestral VSTi's.
If you look at the screenshot below, you will see an extended piano keyboard at the bottom of the picture.
The notes displayed in black and white are the 'playable' sampled notes, the ones shaded pink invoke various articulations, e.g. pizzicato, tremolo, with mutes, etc.
You'll notice these notes lie within the playable range of other instruments e.g. the cello, which uses keyswitches an octave lower.
So substituting one for the other will often lead to playback problems.