This is generally how Social Security classifies jobs based on their physical requirements:

Sedentary work takes place mostly sitting down, but allows lifting up to 10 pounds, occasionally carrying objects (like files), and walking and/or standing up to two hours per day.

Light work allows lifting up to 20 pounds and frequently lifting and carrying up to 10 pounds, with the majority of the day spent standing or walking.

Medium work can require lifting up to 50 pounds and frequently lifting and carrying up to 25 pounds.

Heavy work can require lifting up to 100 pounds and frequently lifting and carrying up to 50.

Very heavy work can require lifting more than 100 pounds and frequently lifting and carrying up to 50 pounds or more.

Some disability claims examiners or medical consultants use another level --  "less than sedentary" work -- for those who can't do what's required of most sedentary jobs. 

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The Importance of RFCs

In deciding what kind of job you can accomplish, considering the limitations of your impairment, the Social Security Administration will prepare a residual functional capacity (RFC) assessment.  An RFC will document the work-related limitations that result from your impairment. The intent of an RFC is to determine what the most is you can do on a regular and continual basis addressing a scheduled work day.

An RFC form submitted by your treating physician can provide incredible substance and insight into your disability claim.  In my opinion this, and it's supporting documentation, can be the golden ring of your application package.  Most applicants believe the relationship between the applicant and the SSA to be an adversarial one.  In reality, the SSA just needs to determine who the truly disabled applicants are.  They want to address their situations as quickly as possible.  A strong RFC form, with supporting documentation, should offer the clarity necessary to expedite the process.  The SSA actually has a Quick disability determination process intended for cases with a high degree of probability that the individual is disabled and also a Compassionate Allowance List for applicants whose medical conditions are so serious that their conditions obviously meet disability standards.  The bulk of applicants won't fall into these two categories, but the existence of the programs alone demonstrates SSA's intent in the process.

If the SSA gives you an RFC for a certain level of work, it means the agency thinks you can do that level of work or less. An RFC for medium work, for example, means that you can medium work, light work, or sedentary work. If your RFC says you can do light work, you can do sedentary work as well. Finally, if your RFC says you can do a sedentary job, the agency will probably conclude there are various sit-down jobs you can do, and your claim will be denied. This is why you must prove that you cannot even perform the requirements of a full range of sedentary work.

If the SSA says you can do "less than sedentary" work, you should be awarded disability benefits.

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Limitations That Prevent Sedentary Work

As an example, here are some physical exertional limitations that can result in an RFC that demonstrates you can’t do the full range of sedentary work:

The inability to lift up to ten pounds.  I can lift 10 lbs., but not repeatedly without experiencing extreme fatigue or exhaustion.

The inability to stand or walk for more than a combined total of two hours a day.  I cannot commit to being able to meet the referenced exertional requirement daily.

The use of medically required hand-held devices to help walking.  I intermittently require a cane or a walker. 

The need to keep one leg elevated.  n/a

The inability to use an arm because of amputation above the elbow.  n/a

The inability to sit for six hours out of an eight hour work-day.  I cannot remain seated for an extended period of time.  My muscles stiffen, sometimes cramping  and need to be exerted by standing, stretching or both.  The frequency varies dramatically.  I also realize spasticity and joint pain (sometimes severe) after sitting for only a short period of time without standing and moving around.

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Here are some examples of non-exertional limitations that can result in an RFC that demonstrates you can’t do the full range of sedentary work:

The need to alternate sitting and standing throughout the day as needed.  I experience this need 24/7.

The need to rest or lie down as needed through the day.  The need to rest, or take a minute to reboot, is an ongoing daily factor in my life.

The significant inability to stoop or bend.  Stooping or bending isn't as much of a problem as is recovering from the stooping or bending.  Getting down to the floor is easy, but getting back up... maybe not so much.  Often times if I bend at the waist and lean forward, I fall forward.  An example would be standing and working at a table or bench.

The reduced ability to use both hands and fingers.  The use of my hands and fingers is excellent for about five minutes and the strength exhausts quickly and they either slow down or no longer work.

You need to take frequent sick days.  Rephrase the statement to say recuperation days and the answer is yes.

The inability to balance even on smooth surfaces.  Absolutely

Certain visual limitations.  Varies

The inability to work in noisy environments. n/a

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How Mental Limitations Can Effect Your Ability to Do Sedentary Work

Having trouble following simple directions and getting work done (and there is proof that this is due to a legitimate mental condition), you might not be able to do any sit-down work. Here are some reasons that disability applicants win their claims based on an inability to do sedentary work.

The applicant took prescribed pain medication that substantially interfered with his ability to focus and concentrate.  n/a

The applicant's condition made it difficult for him/her to perform even simple, routine, unskilled tasks.  The completion of any task in a time sensitive manner could be problematic.  The ability to mentally multi-task is now questionable.

The applicant's condition interfered with his/her ability to complete tasks on time.  The completion of any task in a time sensitive manner could be problematic.

The applicant had difficulty getting along with others or responding appropriately to authority.  I present with the pseudo bulbar affect when addressing emotional or serious issues.  I quickly begin to cry when trying to make a point I feel strongly about or in response to a challenge regarding my performance.  I also inappropriately laugh loudly on occasion, proving both disruptive and embarrassing.

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Limitations That Significantly Erode the Occupational Base

Some limitations, on their own, erode the occupational base to such an extent that the SSA will find you able to do less than sedentary work, and hence disabled. Those limitations are the complete inability to stoop and the requirement to be allowed to sit or stand as needed.

The Complete Inability to Stoop   I can stoop, but I don't recover easily.  This worsens quickly if repeated.

The SSA should find you disabled if your medical records show you cannot stoop at all because of your impairment. This is because almost all jobs, even sedentary ones, require some ability to stoop and bend. The SSA has concluded that this limitation erodes the occupational base to the extent that a finding of disabled is generally warranted.

The Need to Sit and Stand as Needed  I experience this need 24/7.
The SSA should find you disabled if you can prove that your impairment requires that you have the option to sit or stand as needed at your job. This is because there are few jobs that permit an employee this flexibility, particularly unskilled ones. However, if you have this restriction, the SSA will still need to look at facts of your case, such as how often you would need to change positions, how long you would need to stand, and whether the need to alternate positions could be accommodated by normal work-day breaks.

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Additional Factors That May Suggest An Inability To Do Sedentary Work

Your condition requires you to lie down periodically at unscheduled times during the work day.  n/a

You take prescribed pain medication that substantially interferes with your ability to focus and concentrate.  n/a

Your condition causes frequent absenteeism.  This may be an issue, but I'm unsure.

Your condition makes it difficult for you to perform even simple, routine, unskilled tasks.  The completion of any task in a time sensitive manner could be problematic.  The ability to mentally multi-task is now questionable.

Your condition interferes with your ability to complete tasks on time.  The completion of any task in a time sensitive manner could be problematic.

You have difficulty getting along with others or responding appropriately to authority. I present with the pseudo bulbar affect when addressing emotional or serious issues.  I quickly begin to cry when trying to make a point I feel strongly about or in response to a challenge regarding my performance.  I also inappropriately laugh loudly on occasion, proving both disruptive and embarrassing.