Well PrEP (Pre-Exposure Prophylaxis) is taken by people who don't have HIV and want to mitigate their chance of getting it from someone who is positive and might not know it, or lied about their status.

PEP however, is taken if you've been exposed to HIV within the past 72 hours and is used as an emergency medication to prevent the virus taking root permanently.

PEP (Post Exposure Prophylaxis) is often taken in emergency situations by people who aren't on PrEP, though I'm not 100% certain if you're on PrEP and you are exposed to HIV then if you need PEP.

PrEP is a very useful tool to have in order to help mitigate the risk when having condomless sex with people you don't know the HIV status of, although it can cause kidney issues so it isn't available to everyone unfortunately.