At just about any point in the requirements lifecycle, you'll probably have to deal with requirements at different levels of detail. Broader requirements (e.g., vision statements) generally have much less detail than business rules. In either case, you're still accountable for making sure that the details are well documented, that they are reviewed by the right stakeholders, and that they are properly handed off for development, validation or testing, and implementation. Naturally, this process can get really tricky as the number of requirements rises.
In a project that stretches over months, it's particularly likely that the details will vary, either because a lot or a little is known about a solution, because some aspects of a solution are more critical, or because the details vary during the requirements process. How to manage the potentially staggering mass of ever-shifting details?
With the right planning and organizing tools, the process becomes easier. For example, a requirements management plan and a requirements traceability tool (e.g., a requirements traceability matrix), can help you find the order amidst the chaos.
The Requirements Management Plan should spell out the requirements categories that you will be managing, as well as the traceability tools you'll use over the course of the project. Deciding on categories and traceability tools early on will bring you the peace of mind that comes with knowing you'll be able to manage requirements at all levels of detail. For example, you'll be able to refer back to the list of categories to determine if you've taken each category to the appropriate level of detail.
Next, a robust requirements traceability matrix spells out not only how the requirements are linked to each other, but when each requirement is scheduled to pass through the steps of the "elicit-document-review-publish-develop-test-implement" process. If you've taken the time, up front, to plan how you'll categorize the requirements and when you'll flesh-out the details of when they'll be finalized, developed, tested, and implemented, you'll be able to measure your progress and keep a steady grip on managing the requirements regardless of their details. And if you take time throughout the requirements lifecycle to update the plan to reflect the current state, including any changes, you'll have the information at your fingertips to manage the details at any time.