In the UK, per the Royal National Institute for Blind people (RNIB) (
$1) -
* Visual acuity of less than 3 / 60 with a full visual field.
* Visual acuity between 3 / 60 and 6 / 60 with a severe reduction of field of vision, such as tunnel vision.
* Visual acuity of 6 / 60 or above but with a very reduced field of vision, especially if a lot of sight is missing in the lower part of the field.
If you're looking for a more common tern for "severely sight limited", it'd be "legally blind" (since there's still technically some sight regardless of how functional it is) or "blind" (since "visually impaired" has taken on a condescending political correctness to some people, though you wouldn't be wrong if you used it). If there's some other aspect of the classification you need cleared up, please mention what specifically needs clarifying. Otherwise, I hope this helped!