Where does the doctors get their deciding criterion for being diagnosed as legally blind?

Legally blind has many different variances. Most people that are considered blind, can actually see.

My cousins daughter is considered legally blind. She can see, but she is not legally allowed to drive a car because of her poor sight, and she is able to get all the funding available to the blind in Canada.

Where do the doctors get the criteria for diagnosing someone as legally blind? The following link may help explain it a bit better.

https://www.google.ca/url?sa=t&rct=j...10151844,d.cGc

Varies by location but most places in the US use the following. 20/200 is with corrective glasses

1. Legal blindness is defined as visual acuity of 20/200 or worse in the best seeing eye without vision corrective devices, or 20/200 with corrective devices. You cannot claim "blindness" if you will not use corrective vision devices.
2. A visual field that is limited to only 20 degrees.