From FIXwiki

FIX Message SecurityDefinition (MsgType=d)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=d	PreTrade messages - SecuritiesReferenceData	FIX.4.2

The Security Definition message is used for the following:

1. Accept the security defined in a Security Definition message.

2. Accept the security defined in a Security Definition message with changes to the definition and/or identity of the security.

3. Reject the security requested in a Security Definition message.

4. Respond to a request for securities within a specified market segment.

5. Convey comprehensive security definition for all market segments that the security participates in.

6. Convey the security's trading rules that differ from default rules for the market segment.

Message Contents By FIX Version

Notes

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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.