The reason why zones are denoted in Roman Numerals was so that the zone could not be confused with the numerical values of the meter scale----to say Zone 5, for example, could too easily be confused with a luminance reading of 5 on the meter scale, this is specifically stated in The Negative in the piece before the start of the chapters. Of course, the luminance reading of 5 on a particular subject luminance, can certainly be used to determine the camera exposure----- to place that luminance on Zone V. A luminance reading of 3 in a darker area of that same scene, automatically means that area must fall on Zone III with the "3" and the "III", in this example being coincidental, but can never be confused in practice. So a luminance reading of 3 is simply two stops or two zones lower on the gray scale. Once the "placement" is made, all other luminances are read relative to it to visualize where they fall on the gray scale, hince facilitating the final print tones in your mind. It's so easy in application, but often can be convoluted in a written description.