From FIXwiki

FIX Message Component SecurityXML

FIX Specification	Type	First introduced	Deprecated
Common	XMLDataBlock	FIX.5.0

The SecurityXML component is used for carrying security description or definition in an XML format. See "Specifying an FpML product specification from within the FIX Instrument Block" for more information on using this component block with FpML as a guideline.

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.