This is what HMRC says:

By completing and submitting this form, you are declaring that the person named is “disabled” under the terms of VAT law, and that they are thereby entitled to obtain certain goods and services zero-rated for Value Added Tax.

There are penalties for making false declarations. If you are in any doubt as to whether they are eligible you should consult VAT Notice 701/7 (VAT reliefs for disabled people) available on www.hmce.gov.uk or contact the National Advice Service on 0845 010 9000

In practice, they are very flexible – visual impairment without being registered blind still implies being “disabled”