On Birta, a draft is a project's working copy. An agent creates one, empty or copied from an already-published version, and writes its files into it one at a time. Nothing in a draft is live: the project's public address keeps serving whatever was last published, untouched, no matter how many times the draft changes underneath it.
That separation is what makes iteration safe. A visitor to the real site sees none of it until the draft is published by number, which the agent does once it considers the work done. Ask to look first and you get the draft's own address instead: you leave remarks on the page and ask for changes round after round, and it is published when you say go.
A published draft becomes a version; the two are the same thing at different points in the loop.