Copyright laws (and their enforcement!) vary from country to country (a significant consideration on the Wild Wild Web!). International Copyright laws are often hard to enforce (as in "expensive", if not impractical.) But, in theory, and without any agreement or release to the contrary*, intellectual property belongs to the person creating it. We write it, we "own" it.
(* Some sites/boards DO have such a release as part of their basic User Agreement- Atlas does not.)
Now, practically speaking, (and just as if you published a hard-copy book or describe it in a group of friends), once it's out there anyone can steal it, or be inspired by it, and a few tweaks can make it nearly impossible to prove after the fact, if you ever are aware of it.
I once created what I "knew" was a 4th ed AM mage of my own design for a PbP RPG game, but then realized some time later that huge parts of it had been inspired by a long-forgotten reading of some of the Ars on-line material (one magi from Atlas' on-line covenant roster). I could not remember ever reading it, but the parallels were just too close to deny. I had even duplicated the name of the Magus almost perfectly and independently, by scanning thru a latin dictionary until the "appropriate" name had jumped out of the mix. At first, I wondered if some Atlas writer had "stolen" my mage (which had been used in an on-line game and was publically viewable), but the Atlas staff assured me that the material in question had been uploaded long before my character was public, and so I accepted that the fault was my own.
Was it intentional? Nope, it wasn't even conscious! (But that doesn't matter in the eyes of the law.) Was it a copyright infringement? Well, yeah. Did anyone care? No more than they care about any dark elf character with two-scimitars in any RPG, online or not. I wasn't making money on it, nor "publishing" it in any real sense. Did I still "steal" it? Yeah, legally I did, despite the work I put into it- somewhere in my unconscious the work that legally belonged to someone else, that I pretty clearly had read at some point, bubbled up as "my own". Did I still run/enjoy the character? Yep, didn't slow me down a bit. Nor, I think, did it much bother Atlas- they probably would never have known the difference I hadn't asked, it didn't diminish their published material, nor did I profit from it (except in abstract emotional terms). They certainly could have asked me to delete the material, or credit them, or even brought legal action if they wanted - (as if they had the time and money to burn on that pursuit), but they realized that, in this case, it was no biggie, no threat or loss to the material that they in fact owned.
My point? The way things work in theory is not the way that things work in practice. If it's important to you, don't publish it in a casual context. And/or if you do, don't fool yourself that someone won't swipe it or be inspired heavily by it, intentionally or not.