Transistor - Saturation and Active Regions
These are testing points, assuming you have a 1V drop between Vce and 100mA flowing through, you have X amount of gain, so for your particular transistor the BC817, at 100mA being switched, you have a beta between 250 to 600, If your use case is at a different current, the gain may be different (transistors tend to have a fairly wide range of gain in manufacturing) the lowest voltage across the Vce junction is drawn out in Figure 6. when saturated at a given current, it will have that voltage.
Focus on the current, instead of the voltage, the transistor itself will define the voltage, e.g. if your switching from 5V, assuming your switching 500mA, and you don't know where in that beta range you lie, you need atleast 12.5mA (500mA / beta of 40) flowing into the base, The worst case Vbe is about 1V, so 4V needs to be dropped at 12.5mA, meaning a 320 Ohm resistor to satuarate it reliably in all use cases. If your switching a higher current, you would adjust as required
These plots are showing how it behaves when significantly saturated. they only specify this particular case so unless graphed elsewhere, that is all they are promising, in reality the saturation voltage will be fairly low for most transistors, but with how widely the manufacturing tolerance is of units, I presume they did not want to specify anything too close to what a bad unit may not be able to meet.