not every detail is described in an ERD

^ not every detail is described in an Engineering Requirements Doc - ERD

Often it is pretty broad strokes. For a web app, it might describe screens, some use cases, maybe some key choices related to the underlying data model.

Often it will describe how this new change fits into the overall system, how it impacts architecture, how it aligns with longer-term goals of the system, that type of stuff.

Many details are not described, in part because we know they will be discovered during development. There are diminishing returns on the foresight a plan, like an ERD, can give you without actually starting on the work.