While the term seems to originate in the insurance industry (which applies a form of the reasonable rule by determining, for example, whether it is reasonable for a particular medical procedure to be done on a particular client in order to determine if the medical insurance company will pay for that procedure), it applies in many other arenas, including:
In law and law enforcement, the Reasonability Rule is often used to determine the extent of charging a person of a crime, the person's culpability in a tort or civil suit, the appropriate verdict or sentences, and the ultimate questions of "Is justice being done here?", "Does the punishment fit the crime?", and "What is the best way to obtain justice?"
In business, the Reasonability Rule often comes into play in the realm of commerce. Contracts are often signed and executed within the boundaries of the Reasonability Rule (are the terms reasonable to both parties, for example). One can even argue that the "meeting of the minds" requirement under contract law is in fact a codification of the Reasonability Rule.
In education, grading policies generally reflect the Reasonability Rule, most notably on the college and university level: "At what level is it reasonable to expect a class's students to perform?' "Is it reasonable and appropriate to give Wendell an "A" for this course?", "Is it reasonable for Byron not to be penalized if he commits academic dishonesty?", and "Did Byron commit academic dishonesty?" are only a few questions that involve the Reasonability Rule in the decision process.
So how does the reasonability rule apply to Wikipedia?
Editors are urged to observe the Reasonability Rule when working in a massive collaborative effort such as the Electronic Encyclopedia:
Consensus occurs only when the community as a whole agree that a particular action or presentation is reasonable in nature. While the community often has individuals that would disagree with a specific action or determination, it is important that the editors who disagree with consensus are assured that the community reasonably take the differences of opinion in consideration while the process of forming a consensus proceeds. Similarly, it would be unreasonable for an apparent consensus to form that would be contrary to Wikipedia policies (for example, insisting that a material fact is contrary to that presented in reliable sources).
A person insisting on a position or action contrary to the bulk of the community would be violating the Reasonability Rule by repeatedly reverting additions supported by the community itself, for such an insistence is assuming that the community is acting in bad faith; on the other hand, if consensus exists for a particular action or position, insisting that there is no consensus would also be a violation of the Reasonability Rule as it would be unreasonable for the community to assert consensus when none is present. Unfortunately, instances of editors violating the Reasonability Rule are common in Wikipedia and often result in unresolved edit wars, mediation, and more drastic measures by administrators.
Administrators must be diligent in observing the Reasonability Rule when enforcing policy. Is it reasonable to conclude, by using Wikipedia policies, that a particular article should be deleted? Is a particular username a reasonable one for an editor to have, or is it inappropriate for this venue? Is a particular action against a particularly disruptive editor reasonable and appropriate in light of the disruption? Is the action "appropriate" and reasonable in light of the editor's tendencies and proclivities?
Another way of looking at the Reasonability Rule is this: if you're involved in an action or judgment involving (or by) another person, reverse roles. If the role reversal forces a change of opinion as to whether the action or judgment is unreasonable, then the original action—with the original roles—violates the Reasonability Rule. Such violations should be kept to a minimum: full compliance with the Reasonability Rule will result in a minimum of conflict and a maximum of production and enjoyment for all who participate. Such is always the goal of a collaboration of any scale.