Difference between various verifySignatures functions in Corda flow unit testing

What's the difference between those three methods in Corda flow unit testing?

ptx.verify(servicehub)
ptx.verifyRequiredSignatures()
ptx.verifySignaturesExcept(vararg allowedToBeMissed)

Because now I cannot use ptx.verifySignatures() as before and I have those three options! Thanks