If a person doesn't meet the requirements of a Listed Impairment, and if they don't qualify for SSI/SSDI via the GRID Rules, SS will use the 5 Step Sequential Evaluation Process to determine if they can perform prior work or any work. A person than needs to prove their functional limitations prohibit them from performing certain activities. An ALJ will list hypothetical limitations of a hypothetical worker to a Vocational Expert to determine if there are jobs they can perform--unskilled, sedentary work being the least difficult.

One person with a diagnosis might have functional limitations that make it impossible for them to work any job, while another with the same diagnosis is capable of working, so I'm not going to give random examples. You can research the Blue Book if you wish which explains the qualifications required for a claimant to be approved with a Listed Impairment. Anyone that meets the qualifications of a Listed Impairment has such severe functional limitations that it is assumed that they are not capable of working at the SGA level (making $1090 per month).