Two photo scientists, in the 1890’s devised a way to precisely expose films and papers and how to measure the resulting blacking induced when the material is developed. The measure they use is called “density”. This is a numerical value assigned to how much light the blacking retards. The inverse is how much light the blacking allows to pass, this measure is the percent transmission.
As you know, films and papers display a range of tones. Then center of this range is a battleship gray. The gray card is a placard that reflects 18% of the light that is incident to it. Incident is old French meaning, about to happen.
In the mid 1930's, Messrs Jones and Condit at the Kodak Laboratory determined that statistically, a typical sunlit scene integrated to a reflectance value of about 18%. About this time, the Western Electric Company brought to market the first light meter. Kodak Labs publish a recommendation; place a Kodak film box in the scene. Seems the box reflected 18% of the ambient light. Now measure the reflected light from the box top and use this reading to set your exposure.
In 1941, Ansel Adams, a prominent landscape photographer, and his friend, Fred Archer, a photo magazine editor, jointly published the Zone System which provided photographers with a method to precisely fine-tune exposure. Their zone system revolves around the use of an 18% placard (battleship gray). This card replaces the Kodak box top. The 18% gray target became the de facto standard. Today film and paper speed as well as the digital chip are calibrated, and film and digital ISO is established using the 18% gray card.
This shade is unique because if an object that reflects 18% is photographed, the resulting correctly exposed and processed film image of this object will measure 0.75 transmission density units. Additionally, if this film is correctly printed and processed, that object on the print will also measure 0.75 reflected density units. The uniqueness is, object and film and print all measure the same density when exposure, processing, and printing are spot on.
Again, the 0.75 density value is well-thought-out to be the center of the scale of film and paper. This value is the defacto standard used to calibrate instruments used to measure photographic films and papers and this value transfers to light meter calibration also.
The scale of the digital image in 8-bit ansi terms = 128. We can also call this value Zone V in the Ansel Adams Zone System.
If the same scheme is applied, an image of a gray card, if properly repented would likely have a value of approximately 128.