When you try to apply guidances and frameworks from books and struggle to make them work in the real world, consider this perspective: the guidance you find in books are merely guides to touch up and polish what you need to do. They cannot contain everything you need to do. But they do provide advise that helps you polish what you need to do. Whoever wrote the book you are reading doesn't know your particular circumstance, so they absolutely cannot provide specific advice. But they can provide general advice. The value of knowledge contained in books is they expose you to ideas and experience that others have encountered and learned. It is still up to you to apply them appropriately to your situation.

Are Business Rules Requirements?

Business organisations use information systems to streamline their operations. May of the activities that would be done by people are performed by software. The rules that the people would have followed are also embedded in the software so that the software would be able to follow the same rules. An example of such rules is: If the due date for an invoice lands on a weekend, move it to the next working day.

Software is built with reference to requirements that have been identified as needed to make the software useful. Manny of these requirements are functional. They specify what that software should do. For example: Generate an invoice when an order has been shipped. This requirement says that the software must a) generate an invoice, and b) only when an order has been shipped.  

Other businesses using the same software may want the invoice to be generated just before shipment as they will pack the invoice with the shipment.

One different between functional requirements and business requirements is that a functional requirement is about the purpose of the software. Why are we building it. Business rules are about the contraints that the software needs to follow when it does what it does.

Business rules can come from different areas of the business. Some may come from the accounting department. Some may come from the regulatory compliance department. Some may come from the payments and collection department. Or taxation. Or operations. 

What is common is that they are rules that other entities in the organisation also need to follow, whereas the functional requirements are requirements only on the software in question.

Writing is Speaking

From Lucile Vaughan Payne, in her book "The Lively Art of Writing. Understanding Forms"

All writing is the sound of one voice speaking, and all writing can be heard. Every sentence you write, the instant it is read—even when it is read in absolute silence—is heard by the reader as clearly as your actual voice would be heard if you were speaking face to face. When you write, your writing is your voice. Your sound. You.

This is a very important reminder to review your writing and make sure it sounds like your own voice and not in some affected manner.