The Mission. In our naval service an assigned task, coupled with its purpose, is known as a mission. As explained previously (page 48), the purpose indicates the larger aim which is to be served by the execution of the task. The task indicates the assigned objective, i.e., what is to be accomplished; the purpose, the further objective to be served thereby. The word mission is a derivative of the Latin verb, "to send". Its use implies the act of sending someone, or of being sent, as an agent for some special duty, a duty imposed by one in authority. Although an individual, free to do so, may select his own mission, and thereby send himself on a special duty, this is not usually the case where an effective military chain of command exists. Normally the sending authority is the immediate superior; the agent, the immediate subordinate. The mission, once assigned, does not change until it has been accomplished or until it has been modified or revoked by higher authority, usually the immediate superior by whom it was assigned. As previously explained in this connection, the designation of a purpose, linked with a task, is an essential element of a mission as treated herein. It is essential to unity of effort that the purpose of the mission of a commander be common with that of other commanders of the same echelon who are to participate in the effort enjoined by their superior's directives. Directives expressed in the Order Form (page 112 and Chapter VIII) facilitate clear recognition of this purpose, which appears in the general plan of action prescribed in the second paragraph of that form. The commander may consider the relationship thus: My assigned task is to be accomplished for the purpose of carrying out my designated part of my immediate superior's general plan. It is customary to simplify the foregoing to the statement that the mission is: (Task) (statement of the assigned task), (Purpose) in order to assist in the successful execution of (statement of the superior's general plan). The words "assist in", etc., may frequently be understood and therefore omitted. The foregoing expression of a mission affords, as later explained (Chapter VI), a method for clear visualization of the effect desired by higher authority. (See also page 84.) All of his assigned tasks which materially influence the commander's Decision (hereinafter discussed) are properly included in his mission; other tasks, naturally, may be omitted in this connection. In the case of a double or multiple task (page 86), all the tasks may be related to a single purpose, or the included tasks may each, or in certain combinations, be linked separately to appropriate purposes.