Voltage stability factor of voltage regulation circuits
To answer: What does Z1 do?
We'll have to ask ourselves: When will the zenerdiode Z1 start to conduct?
When enough voltage develops across it, for example when the output voltage U2 gets very low, for example when the output is shorted.
In that case Z1 conducts and pulls down the voltage at the connection R1 and R, that will also pull down the voltage at the base of T1. T1 will then draw less current from the base of T2. That will make T2 conduct less so that the current flowing through T2 to the output (I2) is decreased.
This is in essence a short circuit protection that prevents too much current from flowing when the output voltage gets too low, for example due to overloading or a short circuit.
As the protection appears to trigger when there is a large difference between input and output voltage, it could also reduce the current when the input voltage gets too high.
You will have to define what you mean by  voltage stability factor (Sv) as this is not a commonly used term (anymore). Look at the datasheet of a commonly used regulator IC like the LM7805, there you will find &quot;line regulation&quot; which is probably similar to your Sv. Line regulation defines how much the output voltage changes as a result of an input voltage change.