Generally any reference to the "Zero/O" line in the Architectural/drafting/machine part print/etc world is in regard to a 'draftsman's SCALE ruler' (those triangular/3 sided white scale rulers) the "Zero/O" reference line (the "zero" starting line on the scale desired that your measurements will be based on) will vary depending on the print scale deployed, and will be located at different points on the various rule's scales depending on what scale was chosen