I was editing DC Special #27 and some characters are listed as Earth-One and some as New Earth but it's not clear to me why or how someone could know which one to choose. It seems pretty arbitrary. Can break down how/why this is happening? How could characters from Earth-One *also* be interacting with New Earth characters?
I posted a response on that issue's talk page, but a broader discussion is probably needed.
In the the early days of this wiki, it was decided that characters like Barry Allen, Hal Jordan, Arthur Curry, J'onn J'onzz, Oliver Queen and Ray Palmer, to name a few, did not warrant separate Earth-One pages and their New Earth pages would cover their Pre-Crisis and Post-Crisis histories. A similar situation exists for Earth-Two/New Earth JSA members like Jay Garrick and Alan Scott.
Other characters, generally those most altered by the Crisis, got Earth-One and New Earth pages: Kal-El, Bruce Wayne, Diana, Katar Hol, Shayera Thal.
Over the years, some characters that initially only had New Earth pages were split: Barbara Gordon and James Gordon come to mind.
The current unwritten policy is that minor Pre-Crisis characters who are not known to have appeared or been mentioned Post-Crisis should not have (New Earth) in their pagenames. So Radea should probably be (Earth-One).
I don't see a lot of value in having "Earth-One" appearances and "New Earth" appearance for characters. At the very least, we could have a separate article or history section that only covers Earth-One but it seems like a bad idea to randomly split them up like this.
The idea is that most existing characters that appeared again after the Crisis weren't changed enough to warrant a second, separate page. Since they changed so little, if at all, their history sections would be identical to their Earth-One counterparts. It doesn't make sense to duplicate that material on so many pages. The characters who were drastically changed either got separate pages from the start or were split when editors working on those books realized how different they were and split them.
new earth/earth 0 is where all the main stories from rebirth happen, earth one is a more modern world where the only heroes are batman, superman, wonderwoman, green lantern (hal) and teen titans, which are all in their early days.
Side note: Prime earth is everything from crisis on infinite earth to the new 52, and new earth is all the new 52 and rebirth titles