Developmental editing has different meanings for those editing fiction compared to those editing nonfiction. As I say elsewhere on my website, a developmental edit for a nonfiction book about the evolution of Japanese funerary rites requires a different skill set and methodology than a developmental edit for an Afro-futuristic space opera novel.
In fact, the definitions for different levels or types of editorial services are not universally agreed upon by editors in the US—many terms are used interchangeably when they should not be. Editors in the UK, Canada, and Australia seem to have more concrete and widely accepted definitions for each type of editing available, though definitions for those types of editing are not always consistent between countries.
For example, the Editors Canada’s website mentions several types of editing, whereas the SfEP website only lists two kinds of editing (proofreading is not the same as editing, by the way). When you communicate with your industry peers, please understand that your perspective may be vastly different than theirs regarding what a specific editing project should or should not include.
Where can you learn more about developmental editing? Beth Hill’s The Magic of Fiction and Barbara Sjoholm’s An Editor’s Guide to Working with Authors are both book that can help you learn how to approach developmental editing for fiction, though many of those skills are learned best by reading, studying, and practicing fiction. I also recommend reading Scott Norton’s Developmental Editing to learn more about nonfiction editing.