Hi Jaya,
Here's a definition, towards the bottom of the listing...
http://en.wikipedia.org/wiki/Fiducial
Mainly, a 'global' fiducial refers to the outside of the board, usually on the scrap rails. This allows the machine to locate two reference points that are programmed beforehand and uses these reference points to indicate any 'skewing' or angle that the board may be sitting. Then, the machine will calculate all the placements, including any discrepencies to rotation.
Having programmed various machines with different panels, I always like to see 'global' fiducials on the scrap, then 'board' fiducials for any boards within a panelized PCB and then 'local' fiducials, for very difficult to place parts, such as large, fine pitched QFPs.
The terminology may differ from job to job, designer to designer and program to program, but I hope this is enough for you to get started.
Dan
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