Question: How do business rules fit with requirements?

Rules are all around us in the real world - in the games we play, in the laws and regulations of society, in the limits we set for our children - everywhere. Yet for whatever reason, rules have been seldom featured in requirements and IT methodologies. Very strange!

So the very first principle of the Manifesto aims to correct that omission, and by doing so, to bring better balance to requirements ...

1.1 Rules are a first-class citizen of the requirements world.

This principle does not say or suggest that business rules are more important than other requirements - for example, process models - but rather, that they should be co-equal. How can you organize or model any kind of activity without knowing the rules?! This understanding leads to the second principle of the Manifesto ...

1.2 Rules are essential for, and a discrete part of, business models and technology models.

The "discrete part of" in this statement is crucial. It means that rules should not be embedded in other deliverables - for example, use cases - so that the rules can be written once and then applied everywhere (single-sourcing). It also means the rules can be validated directly with business people and subject matter experts. The result is better requirements - and better communication.

Another result is rule independence. The rules can now evolve independently of other architectural components, often much faster. By not hard-coding rules into application programs, much more agile business solutions can be achieved. The Manifesto states the principle this way ...

6.1 A business rules application is intentionally built to accommodate continuous change in business rules. The platform on which the application runs should support such continuous change.

References:

[1] The Manifesto is free, only 2 pages long, translated into 15 languages. Have a quick look (or re-look!). No sign up required. Well worth your time.