Classic Approach

The high-level specification that declares business need is named Business Requirements Document (BRD). Frankly, I saw many cases when it was just a comments thread in a ticket system.

The next possible level is the analysis of an impact to IT systems. If formalized in a document it's the Architectural Document. This is the usually skipped level in mid-size organizations. Who saw clearly documented architecture?

The low-level is detailed requirements made by analysts in the form of Software Requirements Specification or ..., mostly, again, tasks in a ticket system.

So, there are levels rarely followed by the people.

Agile Approach

Everything is in the form of backlog records. Main kinds of it is:

Epics — sort of business requirements. Optional.

Stories — sort of business requirements somewhat detailed.

So, these are levels that can be followed easily, because of simplicity.

Isn't it?