From FIXwiki

FIX Message ContraryIntentionReport (MsgType=BO)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=BO	PostTrade messages - PositionMaintenance	FIX.4.4

The Contrary Intention Report is used for reporting of contrary expiration quantities for Saturday expiring options. This information is required by options exchanges for regulatory purposes.

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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.