The set of rules to be considered at any point of time depend upon the current context of their execution.
As a member of society, for example, you have certain rules of etiquette to consider when you dine at a fine restaurant. These rules might be exclusive to the set of rules you follow when spending a sunny day at the swimming pool. There are other, higher-level rules that apply in the broader context of civilized society, regardless of whether you're at a restaurant or at the pool, such as laws against cannibalism. You wouldn't eat a person if you were at the restaurant or hunt them down if you were at the pool. It's just not done, at least not in the context of civilized society.
One set of rules may be useful in one context, and be replaced with an altogether different set in another context. At the same time, these rules might share common aspects from a higher level context. Within a Rule Engine, a collection of rules that can be applied at runtime is called a rule set. The collection of rule sets applicable given the current context is called a rule base. You might consider a rule base to be a collection of one or more rule sets that apply to a given operating context.
