From FIXwiki

FIX Message SecurityListUpdateReport (MsgType=BK)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=BK	PreTrade messages - SecuritiesReferenceData	FIX.4.4

The Security List Update Report is used for reporting updates to a Contract Security Masterfile. Updates could be due to Corporate Actions or other business events. Update may include additions, modifications and deletions.

Message 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.