As long as i know here is no "WHEN".
Basically, the difference between PNP and NON is that PNP gets a + to the collector and needs 0 to the base to become conductive.
NPN gets 0 to the collector and needs + to become conductive.
This is the main difference. So, if you need to switch for example a relay and you have an output that sends 0 when you need to turn it on, then you must use a PNP transistor.
Also, is very usefull in reversing signals (like amplifiers).
There are no more differences between them as long as i know...