So, I'll start by saying that it's pretty on brand for this meme post lol, but there are multiple definitions of bisexuality and pansexuality I find acceptable. There are some that are bullshit. I don't really subscribe to one true definition of it. I have one I use the most that I'll talk about later in this post. This is what I take issue with, see emphasis.
Bisexual is attracted to more than one gender and CAN include all the same attractions as a Pansexual.
Pansexual IS attraction to all genders.
So yeah all pansexuals are bisexual but not all bisexuals are pansexuals, they could be trans excluding, that's sufficiently vague enough that Pansexual is its own thing.
This is either biphobic by assuming bisexuals are not accepting of trans people or transphobic by viewing trans men as a different gender than cis men (same for trans women and cis women). That's why I have a problem with it. Bisexuality has never meant that.
Someone else in this thread has already mentioned the bisexual manifesto from 1990. Now, to be clear, this isn't me saying "hey, it's called the bisexual manifesto, it's the one true definition of bisexuality." (It itself even says its not.) But I find this part relevant when discussing multiple definitions.
Do not expect each magazine to be representative of all bisexuals, for our diversity is too vast. Do not expect a clear-cut definition of bisexuality to jump out from the pages. We bisexuals tend to define bisexuality in ways that are unique to our own individuality. There are as many definitions of bisexuality as there are bisexuals. Many of us choose not to label ourselves anything at all, and find the word 'bisexual' to be inadequate and too limiting. Do not assume that the opinions expressed are shared by all bisexuals, [...]
So even in 1990 it was understood by bisexuals that bisexuals did not have a single "one true definition" of bisexuality. Not to even mention pansexuality and what shape the venn diagram of the two might make.
Me, personally, the definition I use, that I feel I see many people use and also I have some bias towards because I feel it's the most satisfying is that pansexuals are attracted to traits of people without regards to gender. While bisexuals are attracted to any gender of people and may have traits they find attractive in some genders but not others. And the example I always give of this is, someone who is bi, but not pan, might find muscular women attractive, but dislike muscles on men. Meanwhile, someone who is pan would just like muscles regardless.
Another definition I use, but perhaps less useful to answer you, is that bisexuals are people who identify as bisexual. Pansexuals are people who identify as pansexual. And yes, people can identify as both or only one. I think self identification in the queer community is massively important. I have a friend who's wife came out as nonbinary. He calls himself straight. So when people ask me what his sexuality is, I say he's straight. Is there a part of me that believes in some technical, theoretical sense that he should consider labeling himself as something else? Sure, yeah, because his wife is nonbinary now, but that's not really my business unless he asked me about it.