(Edited as It had been some time since I had looked at the code closely

!
Survivability is a mix of both armoured vehicle damage absorbtion of a penetrating hit
and crew bailout survivability if the vehicle gets destroyed.
It affects armoured vehicle penetrations, when the code determines what happens as the AP round bounces round the inside of the vehicle causing damage (dead men, * points and weapons deletions). The more overpenetration the more chances of killing crew and/or causing damage to weapons etc. The larger the warhead size that penetrated, the worse chance for the crew inside the vehicle.
The survivability number reduces each test a bit. Therefore it has a more noticeable effect if the penetration is not too severe, reducing that to a * or a few **'s instead of what may have been an outright kill had the survivability been a bit lower. It'll have more noticeable effect if the warhead size that penetrated was smaller, of course.
Damage points are not necessarily crewmen killed - the survivability number helps to ensure damage points (*'s) do not convert over into actual crew kills.
A massive overkill (larger warhead size and excess penetration) will of course have more chances to do damage points - the survivability will help here, mainly for men staying alive as there are just going to be so many more tests for damage that the vehicle is going to be doomed unless very lucky.
If all the crew get killed off before the vehicle reaches its max damage points, then it is a catastrophic kill. If the damage point limit for the vehicle is reached before all are dead, the vehicle is killed with a normal bail-out event.
On a total kill, with officially "no survivors", there is a low chance saving throw to see if a man survived the brew up, and that is based on the survivability value. The difference between 0 and 6 makes a difference here, as the basic chance is rather low.
The value is then used to determine success for any surviving crew and passengers bailing out of the vehicle (including the 1 man added back to the crew on a total kill, if he was really lucky).
The bailout code is the only bit that survivability would add effect to
soft vehicles as they are destroyed on 100% crew killed, through the HE hit code, and not via the AP penetration code.
So - it is more than simply crew/pax bail-out, it also is a value for reducing the damage (to both the vehicle and the crew) taken inside of an
armoured vehicle when penetrated.
Cheers
Andy