Transistor Input ON Voltage
Look in more detail at the table: -

There are limits for Input ON voltage and Input OFF voltages. They tell you: -

ON - to ensure at least 2 mA collector current for all the devices you might buy, you need to apply at least 1.4 volts.
OFF - to ensure a collector current of less than 100 uA for all the devices you might buy, you need to apply less than 0.4 volts.


At which point are they mentioning that 0.5V should be? Before or
After R1 in the above diagram?

The pin called &quot;B&quot; is the input pin. There are no specifications for the actual base terminal of the internal transistor because it's &quot;un-touchable&quot; and internal.