One word makes it a separate category, like there are "women" and "transwomen". Two words makes it an adjective, like "tall women" or "fat women" or "blonde women" -- we don't write those as "tallwomen" or "fatwomen" or "blondewomen", because they're not different entities, just descriptors.

Far from being "just terminology", it's an important point that reinforces that trans women are not in any way second-class women or "woman, but with an asterisk": they're women, just like every other woman is, and the "trans" part is just an adjective narrowing down the specific descriptors.

That's the best explanation I've seen; thanks

Oh okay, thank you very much for explaining.

Also, TERFs have very much weaponized "transwoman" as a dog whistle to one another