Final answer:
In vision testing, the numerator indicates the standard testing distance, usually 20 feet, at which the visual acuity is assessed against a Snellen chart.
Step-by-step explanation:
When checking vision, the numerator portion of the visual acuity reading represents the standard distance at which vision is tested, typically 20 feet in the United States. For example, in a 20/60 vision, the numerator '20' indicates the distance in feet between the person being tested and the Snellen chart. A person with 20/20 vision can see at 20 feet what a person with normal acuity should be able to see at the same distance. However, a person with 20/60 vision can only see at 20 feet what a person with normal acuity could see from 60 feet away. Visual acuity tests like the Snellen chart assessment provide a generalization of a person's sharpness of vision, relative to a standard or normal level of acuity.