If your story is already published and approved, you will have both options "Save as draft" and "Publish chapter". Until then, you can only "Save as draft". (If it is your story, the chapter will be public a soon as you hit "publish chapter".) If your story isn't published, you have to publish it first. (Go to the "Edit Story" page of the story. E.g. go to the story map and replace the "map" in the link with "edit".) After you published your story, it needs to be approved by a moderator. This might take some time. As soon as it is approved by a moderator, all existing chapters of the story (no matter if finished or not) will be published with the story. (Currently, there is no way for users to prevent this.)
