MarketDataRequestReject/FIX.5.0SP1+

Message Content of MarketDataRequestReject (MsgType=Y)

TagText	FieldName or ComponentName	Required	Description
	StandardHeader	x	MsgType = Y
262	MDReqID	x	Must refer to the MDReqID of the request.
	Parties		Insert here the set of Parties (firm identification) fields defined in "Common Components of Application Messages
281	MDReqRejReason
	MDRjctGrp
58	Text
354	EncodedTextLen		Must be set if EncodedText field is specified and must immediately precede it.
355	EncodedText		Encoded (non-ASCII characters) representation of the Text field in the encoded format specified via the MessageEncoding field.
	StandardTrailer	x

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.