Writing for yourself is one thing; most anyone can write simply for themselves.
Writing for an audience is a step above that; there, the writer must take into consideration someone outside themselves when they write, but the audience doesn’t actually control what is written.
Writing for someone else when they are paying for it is what makes it professional, especially when they are paying rates that other professionals consider a professional rate. This level of writing is whole level above any of my other writing, because largely it is to someone else’s requirements. Said someone is also a professional, someone with more experience in writing and in editing fiction, and someone stronger and more definite demands and desires for the fiction they purchase.
Myself, I have written fiction professionally, or, in some cases, intended as professional. The following is what I have online at the moment: