As a Trans Women/Enby Pan person here, some people try to set up "rules" for the distinction but it's kinda just a vague vibes thing. The general theme is that Pansexuals are on average attracted to a wider range of people than Bisexuals. But contrary to what some people think bisexual is not limited to the binary genders. Honestly this could also be said for most identities, it's just what label you choose to call yourself.

For instance, I call myself Pansexual because not only do I like both genders (though I lean towards women), androgynous people specifically get to me. It's like both sides of my sexuality light up at the same time.