A 4 Severity injury is 4 Dice applied to the characters C1, C2, or C3 as is mentioned in the BTSD section when Eneri took a light (1D) wound. I agree its not spelled out in big glowing letters and i would like to see more examples but that is my interpretation of the wounding rules and combat mechanics.
I see what you're saying, but I don't agree at all that's what Marc intended with T5.
Here's the way the example you are speaking of works out--
BTSD (page 233) is showing an example of using Bad Flux. The roll for the Bad Flux was a modified -1 for Eneri and a -2 for Aia and a 0 for the ATV.
Looking at the BTSD table on that same page, a roll of -1 equates to Slight damage.
A roll of -2 equates to Light damage.
And, a roll of 0 equates to a Scratch.
But, what is a Scratch? Slight or Light damage?
Turn to the Master tables, page 183. See the one on Table 11 named "Wound"? For the ATV, look at the table named "Damage" on Table 11.
It says...
A slight wound is 1D damage.
A light wound is 2D damage.
The ATV suffered a scratch, which, on the table, is 1 point of damage.
So, Enri takes 1D damage.
Aia takes 2D damage.
The ATV takes 1 point of damage, which, as the example says, is of no real consequence and can be buffed out later.
Severity is only used in Diagnosis and Repair tasks, or when an effect overwhelms protection (such as when Bang damage overwhelms soundproofing to make a character deaf for a number of rounds equal to the Bang damage).
See the Battle Damage tasks on page 232 (and the Medical skill in the skills chapter) to see how Severity is used with tasks.
Wound severity is not used, as you say, to have a player roll damage, find severity, then roll different damage.