FIX Field 544
Identifies whether an order is a margin order or a non-margin order. This is primarily used when sending orders to Japanese exchanges to indicate sell margin or buy to cover. The same tag could be assigned also by buy-side to indicate the intent to sell or buy margin and the sell-side to accept or reject (base on some validation criteria) the margin request.
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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.