There is a bible but it doesn't really impose limits. It's a massive document that details every story, the main characters, technology, ships, etc.
The writer produces a treatment then a draft. Editors, me and who knows who else, read it, suggests changes. Those changes get fed back to the licensee and author. Sometimes the suggestions are adopted (eg. there was a line in Prototype about an egg closing and no seams were visible. I said eggs always have visible seams, and I believe that line got tweaked) and sometimes they're not adopted (eg. earlier in this thread about the use of "Xenomorph").
