Leader and Trailer Units -- The Leader value is the number of units of leader required at the beginning of the machine control data file. The Trailer value is the number of units of trailer you require at the end of the machine control data file. Units are determined by whether you have designated internal postprocessor representation to be in inches or millimeters. (Dblcom 429 and 430 for mills; Relcom 417 and 418 for lathes)