For example, let's say you're known as Tim Patterfoot on Helium. When you get to Associated Content, you decide you want to be known by your real name, Jim Patterfoot. This is completely fine... unless you want to republish your Helium articles on Associated Content or vise versa (click here for content right information which includes how you can have the same article on more than one site). If you want to do that, then your names have to match up on both sites. This prevents stolen articles and increases your credibility.
If you have any questions concerning bylines, then feel free to ask them in the comment section. I'll answer them the best I can and if I don't know the answer, I'll find someone who does.