MLA, APA, and Chicago are style guides for specific subjects and fields—but they can't cover everything.
There are so many fields—and so many publishers—that have created (or adapted) their own styles to suit there needs.
These style guides are just that, guides. Their citation rules are not set in stone because there are too many possible resources to prepare for them all. They are a starting point for you.
Businesses, publishers, and journals all have their own style guides that are variations on this style for what types of writing they need. Likewise, your professor might have specific formats and preferences for the class. Follow your professor's preferences and ask for clarification and specifics as needed. You are also welcome to ask the library for assistance with citation and formatting.