Sometimes employees like to throw up their hands after being put on a PIP and say, "There's nothing I can do now! They're going to fire me no matter what!" but I think that's a cop-out. Once you've been told exactly what you need to do to meet expectations, then the ball is in your court and you own your success or failure.
The reality is that you can fire someone whether or not they've been put on a PIP. A PIP is certainly a demonstration of good faith and it does protect the company from wrongful termination claims to some extent, but it's not a requirement.
Thanks for reading, Brendan!