A frame marking of 50 20 means the:

The marking of a frame as "50 20" indicates that the frame measurements are provided using the boxing system. This system is a method of frame measurement where the numbers represent the overall width of the rectangular shape (50 mm) and the vertical height of the box in which the lens sits (which would require the understanding that the second number relates to the eye size, generally the vertical dimension from the top of the lens to the bottom).

In this instance, the "20" refers to the eye size, which is determined based on the boxing dimensions. The boxing system provides a standardized way to categorize and reference frame sizes and is commonly used in the optical industry for compatibility with prescription lenses.

On the other hand, the datum system involves a different way of marking frames that focuses more on the geometric center and reference lines of the frame rather than an overall boxing dimension. The datum line is typically in reference to measurements and positioning of lenses that accommodate the wearer's ocular alignment, rather than box sizes.

Therefore, understanding that "50 20" reflects measurements in the boxing system is key to comprehending how the dimensions relate specifically to the frame's geometry and how it is utilized in dispensing eyewear.