When denoting test results in Snellen notation, what does the top number represent?

In Snellen notation, the top number represents the distance from which the test is being conducted, expressed in feet. Typically, when someone has their visual acuity tested using the Snellen chart, they are positioned a standard distance—usually 20 feet—away from the chart. The notation helps to indicate the clarity or sharpness of vision. For example, if the notation reads 20/40, it signifies that at a distance of 20 feet, the individual is able to see what a person with normal vision can see at 40 feet. This use of distance is crucial as it standardizes visual acuity measurements and allows for consistent interpretation of results across different individuals and settings.