A drilling program is used to communicate the required holes on a printed circuit board to the PCB manufacturer. The drilling program is exported from as Excellon or Sieb & Meyer from the circuit board layout software and usually accompanies Extended Gerber data (RS-274X).
The drilling program itself is an ASCII text file defined by the IPC NC-349.
Preferable is the Excellon and Sieb & Meyer 3000 (SM3000) format as they already have the aperture table (diameter of the holes) integrated. The format SM1000 is not recommended (extra aperture table).
Since the metric system has become established in the production of circuit boards for drilling, the drilling program should be output as metric data. The formats 3.3 leading mm or 4.4 leading mm have proved their worth. That is, a coordinate is given in mm and has 3 places in front and 3 places after the decimal point at 3,3 leading, and 4 places each at 4,4 leading. The commas are not displayed in the finished drilling program!
Especially with the 2.3 inch resolution, problems with hole positioning can occur if the Gerber data has been output in a higher resolution. For each type of hole (for example, through-plated, non-plated-through, blind holes, ...) a separate drill file should be exported.