Registering copyright is not needed to have the copyright. You have that automatically in the US since 1989 by being the creator and publishing your work somehow also does not hurt to prove it.
Registration only brings some questionable benefits that might even be in violation of the treaty the US signed. Complicated stuff, but in the context we talk about this, almost no indie or amateur dev is gonna register their assets or pieces of work. How does that even work for software versions? Every published version needs their own 35 $ registration or what? Or worse, assets, every separatly "tangible" asset needs registration? Also, this only is valid for the US, afaik. In another country that also signed the treaty, only the fact that you are the copyright holder counts, not the registration in a US registrary.
Copyright only protects the actual works, hence my question about the different software versions. Also, software usually is governed by licences, yet another can of worms.
That is what you get, if you want to apply concepts for physical items to the digital world ;-)