Pop two values and one string, the first value is the new string size (at most that many characters) and the second value is the index (1 based) of the first character to start the copy from. The resulting string is pushed back on the stack.
Since version 5, the String object substr(), substring() or slice() functions should be used instead.
This action acts on bytes, not UTF-8 characters. In other words, it does not work with international text. For that purpose use the SubString (multi-byte) action instead.