28 C.F.R. § 32.23
Direct result of an injury -A disability results directly from an injury if the injury is a substantial factor in bringing the disability about.
Gainful work means full-or part-time activity that actually is compensated or commonly is compensated.
Medical certainty -A fact exists to a degree of medical certainty, when, pursuant to a medical assessment, the fact is demonstrated by convincing evidence.
Permanently disabled -An individual is permanently disabled only if there is a degree of medical certainty (given the current state of medicine in the United States) that his disabled condition-
Product of an injury -Permanent and total disability is produced by a catastrophic injury suffered as a direct and proximate result of a personal injury if the disability is a direct result of the personal injury.
Residual functional capacity means that which an individual still is capable of doing, as shown by medical (and, as appropriate, vocational) assessment, despite a disability.
Totally disabled -An individual is totally disabled only if there is a degree of medical certainty (given the current state of medicine in the United States) that his residual functional capacity is such that he cannot perform any gainful work.
28 C.F.R. §32.23