Preconditions
Preconditions are conditions that must exist before the method can execute. Authentication, for example, is a precondition. I want everyone contacting my service to be authenticated before I do anything. Preconditions are typically things such as system state, arguments, initialization parameters, and other metainformation that is required to be in place before the service is executed.
Preconditions also represent an obligation that the client has to meet before being able to execute the service. Remember this is a contract. The contract has two sides. Take the Yahoo! traffic example. A precondition was to submit a valid application ID before the query would execute.
Finally, if a service request satisfies all the service's requirements, then the request is allowed for the entire system.