http://en.wikipedia.org/wiki/Public_Domain
It means anyone can do whatever they want with it. They can't claim copyright, though, since they didn't create it in the first place. They can modify it an claim copyright on their modifications, at least as long as they're significant enough, but that doesn't affect existing works in the public domain.
> This is kinda what I had pushed for earlier in >>52. I think that separating the sage (aka, "don't bump"), fusianasan (aka, "show IP"), and ID:Heaven (aka, "no ID") functions from any particular post elements in the main scripts would be ideal for implementing Kareha in systems where inputting a certain string to trigger these functions is not intuitive (ie, every board outside of the 2ch/Futaba family). These trigger strings (S_DONTBUMP, S_SHOWIP, S_NOID) and their assignment to a certain form field input could be instead implemented individually in each template.
There's no obvious way to do this, since there has to be code that specifically checks a field and takes certain actions long before the template comes into play. It'd take some sort of plugin system to implement it, and I don't think that's quite called for.
Also >>154 is Kami.