The ability to lift and/or carry is one of the basic building blocks of a worker's Residual Functional Capacity (RFC). However, the focus is almost always on the lifting, and it shouldn't be.
I had a conversation recently with a client with leg problems due to neuropathy. I asked about lifting 20 pounds (required for light exertional work). The client replied "I can lift that, but I can't do anything once I've got it." He could lift but he could not carry.
Several medical conditions (including degenerative joint disease of the hip, knee, or ankle, peripheral neuropathy in the feet, or peripheral vascular disease) can allow a claimant to lift but not carry 20 pounds or more on a sustained basis. A claimant must have the ability to both lift and carry 20 pounds for the light exertional level.
This issue is particularly important for those claimants age 50 or over, since (once past relevant work is precluded at step 4) they must have an RFC at the unskilled sedentary exertional level to prevail under the medical vocational guidelines at step 5.