The RFC assessment is a function-by-function assessment based upon all of the relevant
                  evidence of an individual's ability to do work-related activities. At step 4 of the
                  sequential evaluation process, the RFC must not be expressed initially in terms of
                  the exertional categories of “sedentary,” “light,” “medium,” “heavy,” and “very heavy” work because the first consideration at this step is whether the individual can do
                  past relevant work as they actually performed it.
               
               RFC may be expressed in terms of an exertional category, such as light, if it becomes
                  necessary to assess whether an individual is able to do their past relevant work as
                  it is generally performed in the national economy. However, without the initial function-by-function
                  assessment of the individual's physical and mental capacities, it may not be possible
                  to determine whether the individual is able to do past relevant work as it is generally
                  performed in the national economy because particular occupations may not require all
                  of the exertional and nonexertional demands necessary to do the full range of work
                  at a given exertional level.
               
               At step 5 of the sequential evaluation process, RFC must be expressed in terms of,
                  or related to, the exertional categories when the adjudicator determines whether there
                  is other work the individual can do. However, in order for an individual to do a full
                  range of work at a given exertional level, such as sedentary, the individual must
                  be able to perform substantially all of the exertional and nonexertional functions
                  required in work at that level. Therefore, it is necessary to assess the individual's
                  capacity to perform each of these functions in order to decide which exertional level
                  is appropriate and whether the individual is capable of doing the full range of work
                  contemplated by the exertional level.
               
               Initial failure to consider an individual's ability to perform the specific work-related
                  functions could be critical to the outcome of a case. For example:
               
               
                  - 
                     
                        a.  
                           At step 4 of the sequential evaluation process, it is especially important to determine
                              whether an individual who is at least “closely approaching advanced age” is able to do past relevant work because failure to address this issue at step 4
                              can result in an erroneous finding that the individual is disabled at step 5. It is
                              very important to consider first whether the individual can still do past relevant
                              work as they actually  performed it because individual jobs within an occupational category as performed
                              for particular employers may not entail all of the requirements of the exertional
                              level indicated for that category in the Dictionary
                                 of Occupational Titles and its related volumes.
                            
 
 
- 
                     
                        b.  
                           The opposite result may also occur at step 4 of the sequential evaluation process.
                              When it is found that an individual cannot do past relevant work as they actually
                              performed it, the adjudicator must consider whether the individual can do the work
                              as it is generally performed in the national economy. Again, however, a failure to
                              first make a function-by-function assessment of the individual's limitations or restrictions
                              could result in the adjudicator overlooking some of an individual's limitations or
                              restrictions. This could lead to an incorrect use of an exertional category to find
                              that the individual is able to do past relevant work as it is generally performed
                              and an erroneous finding that the individual is not disabled.
                            
 
 
- 
                     
                        c.  
                           At step 5 of the sequential evaluation process, the same failures could result in
                              an improper application of the rules in appendix 2 to subpart P of the Regulations
                              No. 4 (the “Medical-Vocational Guidelines” ) and could make the difference between a finding of “disabled” and “not disabled.” Without a careful consideration of an individual's functional capacities to support
                              an RFC assessment based on an exertional category, the adjudicator may either overlook
                              limitations or restrictions that would narrow the ranges and types of work an individual
                              may be able to do, or find that the individual has limitations or restrictions that
                              they do not actually have.