First, the simple number of points of damage received by the victim is dictated by the weapon used. The description of weapons includes the damage that they inflict, usually a range specified by a number of six-sided dice with some simple modifiers. The damage is simply rolled. Beyond the clinical and detached measurement of stamina by numbers, this is where the referee could throw in grisly details to help describe the event if they deem it useful. We usually do.
When a character reaches zero (0) stamina, they are on the threshold of consciousness. At negative ten (-10) stamina, the biological character's body is at the limit of final death. Any damage beyond this point means that they are unrecoverable, unsavable (even with the usual advanced technology), and probably pretty unappealing to look at. Mechanical beings are capable of being repairable down to a negative equal to their total stamina.