[petsc-dev] What is the difference between shift and target in SLEPc ?

[Franck Houssen]

What is the difference between shift and target in SLEPc ? Shift (STSetShift) 
is clear to me, but, target (EPSSetTarget) is not. 
Can somebody give an example where one want/need to have a target which would 
be different from the shift ? 

Franck