From FIXwiki

FIX Field 583

Permits order originators to tie together groups of orders in which trades resulting from orders are associated for a specific purpose, for example the calculation of average execution price for a customer or to associate lists submitted to a broker as waves of a larger program trade.

Notes

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Don't forget to refer to the FPL Discussion Groups to back up anything you write.
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Links from FIXwiki pages to relevant discussions on the FPL site are very valuable.

For example if you were writing a clarification on how to specify the "quantity" of an order in FIX, you might add the
following relevant link, http://www.fixprotocol.org/discuss/read/f5014573, to back up your points.