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 <3> 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.