The adjudicators may have also used the General Assessment table...which, not to confuse things, but isn't actually a Table in existence in the Table of Disabilities. It's used to assess as best and fairly as possible those medical conditions which don't fit perfectly into one specific box (chapter). We'll leave that possibility for another time
The assessment of any condition is based on the symptoms. The hardest, in my book, to assess are psychiatric conditions, because so much of the symptomology is subjective - what the patient feels/sees/reacts to. The easiest are musculoskeletal, as we're usually talking about something measurable, like range of motion.
Because a TBI is considered neurological, it would fall under Chapter 20. When there are effects on physical limitations, that's when Chapter 17 or 19 would be used, per the following instructions to Chapter 20, quote:
"• Impairment from brain injury or disease affecting the upper limbs only or the lower limbs only (but not both) is rated within Chapter 17, Musculoskeletal Impairment.")
and
"• Impairment from brain injury or disease resulting only in both upper and lower limb effects is rated within Chapter 19, Activities of Daily Living."