This is what I personally know from my "experience" (reading smut since I was 12-)

First off, Dom and Sub are for both same-sex couples and different-sex couples. Top and Bottom are for SAME-sex couples only as it refers to who is penetrating who (penetrating is such a weird word-). Last I checked, a woman isn't fucking her boyfriend/husband with a dick of her own or a strap on. And if she is, she's probably trans and that's completely okay, doesn't make her any less of a woman, and only then can they use the terms top and bottom.

Second, dom, or dominant, is usually the person who initiates sex, the one who's more seductively and in control, while sub, or submissive, is the opposite, obviously. They submit to their dominant partner and let them control them and do whatever they want (with boundaries, depending on the couple).

As I said before, top and bottom refers to who is penetrating who. So in a male/male relationship, the male who is using his penis to fuck his boyfriend is top, while his boyfriend is bottom. But let's say his boyfriend is riding him. The boyfriend is still bottom, but he's dominant since he is in control.

With a female/female relationship, the woman using the strap-on is, again, the top and her girlfriend is bottom. If her girlfriend is riding the woman with the strap-on, she's still a bottom, but being dominant since, again, she's in control.

There is a difference between dom & sub and top & bottom. It really pisses me off when people don't use the right terms since different-sex couples can physically not be anything other than male is top and female is bottom unless the female is trans and male is cis or the male is trans and female is cis (again, doesn't make them any less of a man or woman).

I could be wrong about some of this, but I'm confident this is the correct information. So if you're writing smut, please use the correct terms. I'm not forcing you, but a straight couple using the terms top & bottom are very incorrect and honestly make me laugh

Can't believe I made an entire book on this, but I did. Yay