OpenChess

what is multi-pv? and when it is used?
thanks
g.

Multi-pv is full analysis of not only what the computer considers the best node, but also the top N-1 alternatives (Where N is the number that multi-pv is set to).
It is useful when you want to force the computer to analyze multiple alternatives. In a normal chess search, only the principle variation is analyzed to full width. The alternative choices are searched with a zero width window.
It can be useful for a number of reasons. One example is that you think an alternative move is better, but you also think that your plan might have a refutation that you have not considered. By forcing multi-pv analysis a program may sometimes see a new plan that was better than it's original plan or it may also show you a refutation that you had not considered.
The possible uses for multi-pv are really limited only by your imagination. I use it for verification of test suites by having multiple engines analyze a position at long time control.