I recently saw a project posted for a developmental editor. Another freelancer posted a comment saying the employer actually wanted a line editor. The employer asked him what he meant.
A developmental editor basically does a rewrite of your work. They are on the edge of the spectrum between ghostwriters (who are usually presented with only vague ideas that are frequently not even written down) and line editors, who do a little bit more than just catching spelling, grammar, and formatting errors. You would send a rough draft of a novel (really rough draft) to a developmental editor and they would basically rewrite it for you to make your story work.
When I, personally, do developmental editing (content editing), I recommend the employer let me do it without tracking changes. Normally, I find a lot of errors on a page, which makes for a very red page that I return. However, the developmental edit would end up almost entirely red. Tracking changes is not helpful because if I move pieces of the work around and then go in to edit them, they will already be red and there is no way for the original author to discern what I have edited and what I have not.
A line editor also may make edits for flow, but not to the extend that a developmental editor does. The line editor also check for spelling, grammar, and formatting inconsistencies. It is important to have the work line edited by a second person after it was developmentally edited by someone. Ideally, you would then send it to a proofreader.
Most writers - especially newer ones with little reading experience and few hours logged in a writing classroom - need to have a developmental editor as well as a line editor.
In the case of the project, the project description (sans title and keywords) was a description of a line editor. However, once you opened the work (which the description told you to do), you knew it was a developmental edit as listed.
Having one or two run-on sentences in your first paragraph (that remains basically coherent) requires a line editor. Having entire pages of wandering thoughts without punctuation requires a developmental edit.