Does the survivabilty stat of a city only apply to itself?

[OxfordGuy3]

Hi - I was wondering, does the survivabilty stat of a city apply only to itself, and/or does it get added to the survivabilty of defending units? Or are the only affects on defending units the defence and armour modifiers. (and prevention of retreats etc.). I assume these two factors are always in effect, even if the city goes into the red and that the only negative affect on units is supply/reinforcement, if cut off from other supply sources?