A C-U always introduces an argument.
If a numeric argument is just a string of C-U characters, then the
value of the argument is
4(numberofcharacters).
Therefore C-U is 4, C-U C-U is 16,
C-U C-U C-U is 256, and so on.
Some commands
care if an argument is present, but don't look at its value; the C-U
character is a good way to say ``here is an argument''.
If there is a signed decimal number after the string of C-U characters it specifies the value of the argument. In this case the number of C-U characters does not matter. An argument of 10 can be represented by C-U 10, C-U C-U 10, or even C-U C-U C-U C-U 10.