"It depends" is one of those phrases at the start of a response that indexers will typically give each other when faced with questions about what to do in a certain situation.

It's a fun answer to give because of the endless possibilities it implies but it is also a serious answer, because so many decisions for how to index a concept or how to format an entry depend on the context of the document or the target audience...or client preferences.

• How should the metatopic be indexed? — Or should it be indexed?

• How should notes be indexed?
  — Or should they even be indexed at all?

• According to what rules should should the index be sorted?

• Should subheadings be listed in the standard, alphabetical order or should they be listed chronologically?

• Should subheadings be run-in (paragraph) style or indented?

• Should images and/or captions be indexed?

• Should two terms be linked by cross references?

• How should two people with the same name be indexed?
  — Or…how should one person with an unknown or only partially known name be indexed?

• How should foreign names be treated — does “Van Gogh, Vincent” get sorted under “Van” or “Gogh”?

These are just some questions an indexer might ask and none of them have really definitive “100% of the time, this is what you do” answers.

There are guidelines, and each publishing house may have its own style guide that will address some of those things.

There are also indexing best practices, which I and many other people talk about in blog posts and books, as well as indexing coursework.

But, there is also this very common refrain that I’ve heard many times, both as a tongue-in-cheek response but also in all seriousness:

It depends.

It depends on context — how the document treats a topic or is organized, and the type of document being indexed.

It depends on the target audience (aka the readers).

It depends on the client’s wishes, whether that is the author, editor, or publisher.

It may also depend on space limitations or time constraints.