The Interpreter Instance API is defined by the RexxInstance interface vector. The RexxInstance defines methods that affect the global state of the interpreter instance. Most of the instance APIs can be called from any thread without requiring any extra steps to access the instance. The two most important instance operations are AttachThread() and Terminate(). AttachThread() allows additional externally identified threads to be included in the interpreter instance threadpool. AttachThread returns a RexxThreadContext interface vector that enables a wider range of capability for the attached thread. The Terminate() API shuts down an interpreter instance when it is no longer needed.