FIX Field 793
Secondary allocation identifier. Unlike the AllocID (70), this can be shared across a number of allocation instruction or allocation report messages, thereby making it possible to pass an identifier for an original allocation message on multiple messages (e.g. from one party to a second to a third, across cancel and replace messages etc.).
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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.