It works more than a counting sequence. The number 0 is defined as '', the number one is therefore 'a' and two becomes 'b' and so on. And so on. Internally the sign is stored and honoured, only on conversation to string it is lost. The charset given upon creation need not be a 'simple' set consisting of all the letters.
Arguments are the value, and optional charset.
This will save you memory and computing power. The length defaults to 1. The length defaults to 1. In this case you need to construct the string to find out the actual length in bytes. Since this omitts the sign, you can not distinguish between negative and positiv values.