Omnisexual is a sublabel of bisexual, similar to pansexual. The main difference between omni and pan is that omni people do have preferences based on genders, or at the very least aren't "gender blind."
Another definition I use though is that gender affects the attraction. It isn't a flat out preference though. The best example I can think of is like... Say there are two masculine women. The fact Woman A is a woman may increase the attraction felt, while the fact Woman B is a woman may decrease the attraction, even though they are both masc (so it isn't just "Oh I have a pref for masc women"). Another way the sexual attraction can vary is based off the actions you'd want to do based off the person's gender.
That's... really about it. While I'm here, though, I want to clear up a few things about pansexuality.
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Pansexuality is not "bi but with trans and nb", nor is it "hearts not parts". Pansexuality is simply a sublabel of bisexuality that means have no preference. Of course, this label is optional. If you have no preferences, you can still just call yourself bisexual, because you are bisexual. Pansexual is just there if you want to specify you have no preference. Omnisexual is there if you want to specify you do have preferences.
Bisexual has always included trans and nonbinary people. Using the "bi = 2" argument is dumb, just like the "pan = all including animals and objects" is also dumb. Bisexual means you are attracted to 2+ genders. That's it. Pansexual means the same thing, just they're attracted to all genders + no preference. Pansexual people aren't better for their lack of preference.
You are allowed to dislike and correct people who use the "bi but with trans and nb", however people have their own reasons for using pansexual and they are not inherently transphobic or biphobic. Not every pan thinks that way. If you truly believe pansexual is the worst thing in the world, though, be kind when correcting people. No one is going to listen to you if you immediately start attacking tgem. Most people who use pansexual are young and learning, and they'll be less likely to change if you're a prick.
Oh and some icons and merch and ACNH stuff
I've made some other carrds too I reccomend checking out