From FIXwiki

FIX Message TradeCaptureReportRequestAck (MsgType=AQ)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=AQ	PostTrade messages - TradeCapture	FIX.4.4

The Trade Capture Request Ack message is used to:

• Provide an acknowledgement to a Trade Capture Report Request in the case where the Trade Capture Report Request is used to specify a subscription or delivery of reports via an out-of-band ResponseTransmissionMethod.

• Provide an acknowledgement to a Trade Capture Report Request in the case when the return of the Trade Capture Reports matching that request will be delayed or delivered asynchronously. This is useful in distributed trading system environments.

• Indicate that no trades were found that matched the selection criteria specified on the Trade Capture Report Request

• The Trade Capture Request was invalid for some business reason, such as request is not authorized, invalid or unknown instrument, party, trading session, etc.

NOTE: A Trade Capture Report Request Ack is not required if one or more Trade Capture Reports will be returned in-band immediately.

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.