Understanding Electronic Schematics

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While design an electronic systems, when to use PNP transistor? When to use NPN transistor?

some time I see in Schematic Diagram, they use PNP transistor some time use NPN. Really don’t understand. Is PNP save more power?

PNP will only function when you apply a negative voltage to the base, NPN will work when a positive voltage is applied to the base.

Automotive Electric Wiring Diagrams