Social Security must consider your combined impairments to determine whether or not you qualify for disability benefits.
At step 4 of the sequential evaluation process, Social Security evaluates your ability to do your past relevant work. At step 5, the SSA considers evaluates your ability to do other work.
You can use different impairments for steps 4 and 5. This is particularly important for those claimants age 55 and over.
Social Security treats people differently depending upon their past work, or vocational background. Sometimes one specific functional limitation will preclude your past relevant work, but would not preclude other jobs.
For example, a limitation to simple instructions, as a result of anxiety or depression, precludes most office work or other semi-skilled work. Then at step 5, a person age 55 or over with physical impairments may be able to rely on the medical-vocational guidelines to demonstrate disability.
Just keep in mind that the limitations that rule out other work at step 5 may be quite different than those that rule out your past relevant work at step 4. That is why Social Security's duty to evaluate the combination of your impairments is so important.