From FIXwiki

FIX Message Component DiscretionInstructions

FIX Specification	Type	First introduced	Deprecated
Common	Block	FIX.4.4

The presence of DiscretionInstructions component block on an order indicates that the trader wishes to display one price but will accept trades at another price.

Component Contents By FIX Version

Notes

Please post comments, clarifications, examples here. Click on the edit button on the right and away you go! 

Don't forget to refer to the FPL Discussion Groups to back up anything you write.
You will often find the authoritative statements on matters of FIX specification interpretation there.
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.