Biual or Pan-ual? What's the difference?

I've always considered myself bi, because I am attracted to men and women. However, in the past I have dated a transual, and a cross-dressing man. I've seen a lot of questions on here about pan-ualism, and I still don't get it. I thought being bi covered all grounds. I still say I'm bi, even though I'm attracted to MTF, and honestly, this is the first I've heard about pan-ualism. What's the difference?