A Type-On master unit, designated as the P205 grade spirit master consists of three parts. The top sheet is a white 45# bond paper. This is called the "master" sheet. It is glued to a second sheet of paper that has a solid coating of purple ink on one side, facing the master sheet. The ink-coated sheet (called the carbon sheet) has a thin wax coating on top of the ink to prevent smearing. These two sheets of paper, the plain white master sheet and the ink-coated carbon sheet, are separated from each other by a removable tan-colored "tissue" sheet.

The key to using either the P205 type-on units or the Reprofax units is to first transfer the ink from the carbon sheet to the white master sheet. Using the P205 type-on units, this ink transfer occurs through pressure. By typing, writing, or drawing on the white "master" sheet, ink releases from the carbon sheet below it and adheres to the master sheet in a mirror image of what was either typed or written.

The main difference between Reprofax units and P205 spirit master units consists in how they are imaged, that is, in how the ink is transferred from the ink-coated paper or film to the white master sheet. As noted above, the ink is transferred by pressure when using the P205 units. When using Reprofax units, the ink is transferred from the film to the master sheet through a heat process.