Quite a few numbers monitors have asked what the purpose of cut numbers sent in Morse code are. To understand why cut numbers are used, we should first take a look at the Morse characters for each number:

1 - DI DAH DAH DAH DAH
2 - DI DI DAH DAH DAH
3 - DI DI DI DAH DAH
4 - DI DI DI DI DAH
5 - DI DI DI DI DIT
6 - DAH DI DI DIT
7 - DAH DAH DI DI DIT
8 - DAH DAH DAH DI DIT
9 - DAH DAH DAH DAH DIT
0 - DAH DAH DAH DAH DAH

Numbers are the longest Morse characters (excluding punctuation) with each character consisting of five elements, each element consisting of a single dit or dah. When you are sending a message consisting of numbers, it is convenient to shorten some of the characters to save time. This is why the ex-KGB stations use a "T" instead of "0." It makes sense to only send one dah instead of five when the recipient knows that it stands for 0 instead of T. When you look at the numbers 1-3, it's obvious that the only difference in the Morse characters is in the first three elements. And for numbers 8-0, the only difference is in the last three elements. Therefore, if you want to save time when sending a long string of numbers, you can shorten them by "cutting" part of each character off. Since 5 consists of all dits, and 0 of all dahs, we can shorten them do one dit & one dah, respectively. If we cut the last three elements of numbers 1-3, and the first three elements of 7-9, we will be left with the following characters:

1234567890
AUV4E6BNT

This is a cut number system that is in use, although it hasn't been seen much recently. Another cut number system I have seen uses the following:

1234567890
AU34567DNT

This is a variation of what we have seen, without changing the middle five numbers. This works a little better in my opinion because there's no chance that V will be mistaken for 4, and B mistaken for 6, which are common errors.

The system in use by the Cuban stations, however doesn't follow this system at all, and could possibly be a simple letter-number substitution. The characters are as follows:

1234567890
ANDUWRIGMT

These are the shortest Morse characters, excluding E, so they were probably chosen for this purpose. Personally I think it makes it more confusing to get the numbers right using this system than if they sent whole numbers.

There may be a wide variety of cut number systems in use around the world, but these are the most common. Cut numbers are not only used by numbers stations, but by various military stations as well. I hope this gives you an understanding of what cut numbers are all about.