From AJ8B

If you spent any time on the CW bands, or, more specifically, on the air during the
recent CQWW CW DX Contest, you may have heard some unfamiliar CW characters, or
questioned the usage of characters that you knew. More than likely, you heard an
experienced operator using a “Cut” number, a “Prosign”, or an abbreviation. For instance,
you may have worked someone who said that your signal was 5NN. You probably guess that
they were sending 599 to you, however, they were using a Cut number, or an agreed upon
shorthand for the number 9.
Cut Numbers – You may think that cutting the number 9 from “- - - - .” to “-.” is not a big deal,
and you would be correct if you were just working a couple of folks. However, a DX station
who may work 200 or more hams during an on the air session may disagree. Propagation
doesn’t always stay open and is shifting all the time. If the DX station can save a bit of work
per QSO, it adds up! Cut Numbers are used most during a contest where the DX station is
focused on rate more than saving time. The stations who are trying to work him are not
going to hang around for minutes and the more QSOs he can squeeze into a small
timeframe, the better.
In addition to using Cut Numbers for RST, they are also common to use for the
stations zone designation or power output. For instance, I worked OM2VL (Slovak Republic)
in CQ Zone 15 during the recent contest. The exchange from Vegh was AJ8B TU 5NN AE K
which combines Cut Numbers and abbreviations. Vegh sent my call, (AJ8B), Thanks (TU), my
signal report – 599 (5NN), his CQ Zone 15 (AE) and the K to terminate the data sent. Then
he would go on to the next station. Vegh has almost 300,000 QSOs to his credit, so, I am
sure that he leans heavily on these concepts. You need to be careful when and where you
use a cut number. The person copying you should be expecting a number, or they will treat
what you send as a letter and will get confused. That is why Vegh sent the 5 in the 5NN
sequence instead of the Cut Number equivalent. A table of cut numbers is below:

Number     1 2 3 4 5 6 7 8 9 0
Cut Number A U V 4 E 6 G D N T