38 C.F.R. § 4.16

§ 4.16 Total disability ratings for compensation based on unemployability of the individual.

(a) Total disability ratings for compensation may be assigned, where the schedular rating is less than total, when the disabled person is, in the judgment of the rating agency, unable to secure or follow a substantially gainful occupation as a result of service-connected disabilities: Provided That, if there is only one such disability, this disability shall be ratable at 60 percent or more, and that, if there are two or more disabilities, there shall be at least one disability ratable at 40 percent or more, and sufficient additional disability to bring the combined rating to 70 percent or more. For the above purpose of one 60 percent disability, or one 40 percent disability in combination, the following will be considered as one disability: (1) Disabilities of one or both upper extremities, or of one or both lower extremities, including the bilateral factor, if applicable, (2) disabilities resulting from common etiology or a single accident, (3) disabilities affecting a single body system, e.g. orthopedic, digestive, respiratory, cardiovascular-renal, neuropsychiatric, (4) multiple injuries incurred in action, or (5) multiple disabilities incurred as a prisoner of war. It is provided further that the existence or degree of nonservice-connected disabilities or previous unemployability status will be disregarded where the percentages referred to in this paragraph for the service-connected disability or disabilities are met and in the judgment of the rating agency such service-connected disabilities render the veteran unemployable. Marginal employment shall not be considered substantially gainful employment. For purposes of this section, marginal employment generally shall be deemed to exist when a veteran's earned annual income does not exceed the amount established by the U.S. Department of Commerce, Bureau of the Census, as the poverty threshold for one person. Marginal employment may also be held to exist, on a facts found basis (includes but is not limited to employment in a protected environment such as a family business or sheltered workshop), when earned annual income exceeds the poverty threshold. Consideration shall be given in all claims to the nature of the employment and the reason for termination.
(b) It is the established policy of the Department of Veterans Affairs that all veterans who are unable to secure and follow a substantially gainful occupation by reason of service-connected disabilities shall be rated totally disabled. Therefore, rating boards should submit to the Director, Compensation and Pension Service, for extra-schedular consideration all cases of veterans who are unemployable by reason of service-connected disabilities, but who fail to meet the percentage standards set forth in paragraph (a) of this section. The rating board will include a full statement as to the veteran's service-connected disabilities, employment history, educational and vocational attainment and all other factors having a bearing on the issue.
Total disability Individual Unemployability
The key to entitlement of 100% disability is establishing that service connected disabilities render the Veteran incapable of obtaining gainful employment.  This seems more simple than reality dictates.  Often Veterans are denied TDIU because of their ability to perform "sedentary work."  However, the VA is often incorrect.  The VA must base its decision on a careful examination of medical evidence.  Additionally, "sedentary work" has a carefully crafted legal definition relying in part on the Social Security Administration's own definition.  
If you feel you qualify or were wrongfully denied TDIU, put a team of legal professionals on your side who have the experience and knowledge to insure your rights are protected.