Sex, Gender Identity and Orientation Summarized here fairly well for the short version
(With a noteworthy side-note that many people cannot be so easily graphed, not a terrible beginners guide. There are as many genders as there are people after all, some are unsummerizable.)
One cannot know a person better than they know themselves. When it comes to something as personal and in their head as who they are and what they feel like, they are the expert. Every individual is the expert on their orientation and sexuality.
Other people can be opportunities to learn, and teachers, but they can’t know someone elses feelings.
This information is here to show how I as a fairly involved queer have seen individuals and sex/body/human-positive groups use them. This isn’t mathematics, there aren’t clean answers, you can’t plug in a certain amount of factors and then have a perfect clean answer for what someone is that works every time. People are complex, if they say they prefer one term over another, use their preferred term. Use the pronouns they ask for. (That is, if you are interested in being polite of course. Your life, you can be whatever you want. Asshole included.)
I am poly, a genderfluid Androgyne, panromantic and panssexual, (or queer for short) but I am also not an expert. I will do what I can to approach these things respectfully and speak from my own experience, while still trying to teach a little bit of what I learned.
Feel free to suggest Edits and feel free to discuss! I love to learn more about this stuff!