Establishing a service connection generally requires three elements: ''(1) the existence of a present disability; (2) in-service incurrence or aggravation of a disease or injury; and (3) a causal relationship between the present disability and the disease or injury incurred or aggravated during service'-the so-called 'nexus' requirement.' Congress has enacted presumptive service connection laws to protect veterans who faced exposure to chemical toxins during service but would find it difficult or impossible to satisfy the obligation to prove a 'nexus' between their exposure to toxins and their disease or injury. The Agent Orange Act established a framework for the adjudication of disability compensation claims for Vietnam War veterans with diseases medically linked to herbicide exposure in the Republic of Vietnam during the Vietnam War. Any veteran who 'served in the Republic of Vietnam' during the Vietnam era and who suffers from any of certain designated diseases 'shall be presumed to have been exposed during such service' to herbicides 'unless there is affirmative evidence to establish that the veteran was not exposed.' The VA issued regulations establishing presumptive service connection for certain diseases associated with exposure to herbicides in Vietnam. The VA concluded that the statute and regulation do not authorize presumptive service connection for those veterans serving in the open waters surrounding Vietnam-known as 'Blue Water' veterans. The VA decided not just to exclude open water service from the definition of service in the 'Republic of Vietnam,' but to also exclude those veterans who served in bays, harbors, and ports of Vietnam from presumptive service connection. Absent documented service on the landmass of Vietnam or in its 'inland waterways'-defined as rivers and streams ending at the mouth of the river or stream, and excluding any larger bodies of water into which those inland waters flow-the VA has concluded that no presumptive service connection is to be applied. The VA did not implement this additional restriction by way of notice and comment regulation as it did its open waters restriction, and it has not published its view on this issue in the Federal Register. The VA has incorporated this new restriction into the M21-1 Manual, which directs VA adjudicators regarding the proper handling of disability claims from Vietnam-era veterans. The M21-1 Manual 'is an internal manual used to convey guidance to VA adjudicators.' P filed a claim for disability compensation for a number of medical conditions allegedly arising out of his naval service in Da Nang Harbor. The VA denied P's claim and P appealed to the U.S. Court of Appeals for Veterans Claims. The Veterans Court concluded that the VA's definition of 'inland waterway' was 'both inconsistent with the regulatory purpose and irrational,' in part because the VA had offered no meaningful explanation for why it classified some bays as inland waterways but not others. The Veterans Court remanded the matter. On remand, the VA concluded that, because 'Agent Orange was not sprayed over Vietnam's offshore waters,' the VA did 'not have medical or scientific evidence to support a presumption of exposure for service on the offshore open waters,' which it defined as 'the high seas and any coastal or other water feature, such as a bay, inlet, or harbor, containing salty or brackish water and subject to regular tidal influence.' The VA published a 'Memorandum of Changes' announcing a change in policy and an accompanying revision of the M21-1 Manual. The VA instructed its adjudicators to exclude all service in ports, harbors, and bays from presumptive service connection, rather than service in only some of those waterways. P seeks review of this revision pursuant to 38 U.S.C. §502.