Methodsrepresent the function calls of Objects. They are defined in OPC 10000-3. Methodsare invoked and return after completion, whether successful or unsuccessful. Execution times for Methodsmay vary, depending on the function they are performing.

The Method Service Setdefines the means to invoke Methods. A Methodis always a component of an Object. Discovery is provided through the browse and query Services. Clientsdiscover the Methodssupported by a Serverby browsing for the owning Objectsthat identify their supported Methods.

Because Methodsmay control some aspect of plant operations, method invocation may depend on environmental or other conditions. This may be especially true when attempting to re-invoke a Methodimmediately after it has completed execution. Conditions that are required to invoke the Methodmay not yet have returned to the state that permits the Methodto start again. In addition, some Methodsmay be capable of supporting concurrent invocations, while others may have a single invocation executing at a given time.