These option switches are used to set non-standard drawing sizes. For example, for a drawing that is 300mm x 400mm , set OS 15 = 300 and OS 16 = 400.
Any settings in OS 15 and OS 16 will cause the settings in OS 14 Positions 1 and 2 to be ignored. However, settings in OS 14 Positions 3 and 4 are not ignored. This means that "direct' PostScript files of a non-standard drawing size can be produced at a reduced scale in the same way that standard drawings can be.