Note this is copied from the SOP and the entire document can be viewed at http://www.qsl.net/w8wn/hscw/papers/hscw-sop.html

MISSING INFORMATION:
On High-Speed CW MS operation, it is possible to request a missing piece of information.
Note - this does not work as well for FSK441, but could possibly be used. (On FSK441, it's best to just ask for the needed information as these letters are likely to be interpreted as only gibberish).

BBB - Both call signs needed
MMM - My call sign needed
YYY - Your call sign needed
SSS - Report (or whatever report/information exchange used) needed (some have suggested GGG for needing the grid)
UUU - Ur keying is unreadable
(Use "U" when needed. Remember that the other station cannot monitor his keying).
These "requests for repeat" letters are used only when the other station mis-copies something and jumps ahead in the sequence. Thus, they are seldom needed (but very valuable at that time).
When these are used, nothing but the appropriate string of letters is sent. (E.g., "YYYYYYYYYY").
The other operator should respond by sending only the requested information. (E.g., "W8WN W8WN W8WN").
When the requesting operator has the needed data, he returns again to the proper exchange sequence.