From FIXwiki

FIX Message OrderMassActionReport (MsgType=BZ)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=BZ	Trade messages - OrderMassHandling	FIX.5.0

The Order Mass Action Report is used to acknowledge an Order Mass Action Request. Note that each affected order that is suspended or released or canceled is acknowledged with a separate Execution Report for each order.

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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.