Elektron Musical Instruments

For those of you who use it in your tracks, can someone clue me in to 
the use of 'shuffle'. The whys, the hows..

If you had a series of 16th notes, which ones would have their timing 
deviate from the bpm? How does the timing relate to the percentages 
commonly seen on gear?

Do you normally have a single shuffle value for the song as a whole, or 
is there some benefit to having simultaneous patches/instruments/tracks 
on different shuffle settings?

thanks,
Mark