Why we use PNP power transistor in Isolation circuit?

Why PNP transistors are used in Isolation circuit. The signal from optocoupler is first given to PNP power transistor before giving it to Relay. Why is it so?Why we use PNP power transistor in Isolation circuit?
It depends on your power supply's polarity. If the voltage is reversed, then you will need to use NPN. Most people are positive thinking, thus PNP, while everything should work just as well with reversed voltages.





The thinking that positive should be higher than negative (which is mostly the ground reference) comes from western culture thinking, of positive being good and negative being bad. There is no law of physics that says you can't reverse everything and have everything work that way. In fact, that's the way all telephone network plants work, i.e. in negative voltages. For telephone network, which works at -48 volts, the laws of physics apply. Big batteries have the positive terminal as the housing that holds the acid, thus a battery that sits on the ground should have the positive terminal grounded. In fact, car batteries should have worked this way, but western culture and ill-logical thinking prevailed. Also look at most batteries, especially button type, the positive is the container case.