The Gerber format was originally used for the control of photo plotters. Such systems had a light source with a shutter over a so called aperture wheel, a disk with different openings in form and size. While moving the plotter table the shutter was open or closed, the light through the aperture wheel opening exposed the film.Today the photo plotters are replaced by laser plotters.
According to this kind of usage the Gerber format is a list of coordinates together with the information for the shutter mode and the position on the aperture wheel, the d-codes. There is no description for the form or size of the openings in the aperture wheel. Therefore a separate aperture file is needed, where the openings are described (see example).