Part of two-part SSI eligibility test. 

Adults must establish they cannot work at substantial level. 

Children under 18 must establish severe deficits in functional activities at school and at home. 

Both adults and children can establish disability or statutory blindness with medical evidence alone if they meet criteria for a listed impairment.  See https://www.ssa.gov/disability/professionals/bluebook/AdultListings.htm or https://www.ssa.gov/disability/professionals/bluebook/ChildhoodListings.htm.