This is an integer value that indicates the number of the controller register that will store the length offset value. For many controllers this is the "H" number.
The Length Offset value is the Z distance between the tip of the tool at machine zero and the programmed zero. It is measured by the operator with the machine before machining begins. Most controllers automatically enter this measured value into the register identified by the Length Offset value. It is best to make Length Offset equal to Diameter Offset. This is not a length measurement.