The Joint Chief of Staff (JCS) publications and related Terminology Repository (DOD Dictionary) are the most authoritative place to pull those definitions from and is where DAU defers to in most cases. Initially, you should start at the JCS Publications website: https://www.jcs.mil/Doctrine/ and that will lead you to the JCS’s Joint Electronic Library (JEL). The first link should get you where you need to go.
Some of the terms referenced are fairly broad in nature, but some can be pulled from the context of the JCS publication:
Policy, as it pertains to Joint Doctrine is defined as that which, “…directs and assigns tasks, prescribes desired capabilities, and provides guidance for ensuring the Armed Forces of the United States are prepared to perform their assigned roles. Implicitly, policy can create new roles and requirements for new capabilities.” (DOD Dictionary, pg 1-2)
Plans exist at many levels and have differing purposes. Defining a plan at any level other than that which is used by your office would be less than helpful. In general, a plan is “a scheme or method of acting, doing, proceeding, making, etc., developed in advance” (i.e., battle plans, evasion plan…).
Standard Operating Procedure (SOP) is defined as, "a set of instructions applicable to those features of operations that lend themselves to a definite or standardized procedure without loss of effectiveness.” (JP 3-31)
· In a broader definition, procedure is defined as, “standard, detailed steps that prescribe how to perform specific tasks. See also tactics; techniques. (CJCSM 5120.01)”
In plain language, a policy is general guidance or direction and may direct you to have several plans for a number of scenarios, a plan is a pre-defined course of action that may exist at multiple levels, and an SOP is a detailed step-by-step set of tasks to dictate how to carry out a plan.