URL officers will transition into Staff Corps communities, most often the Medical Corps or the Judge Advocate General's Corps following completion of Navy-funded medical school or law school, while others move into other RL or Staff Corps fields. The remaining officers are directly commissioned as RL or Staff Corps. RL and Staff Corps officers are authorized to command ashore within their particular speciality, but are not eligible for combatant command at sea, which remains strictly within the purview of URL officers. Limited duty officers and chief warrant officers In contrast to the U.S. Navy's limited duty officers (LDO) and chief warrant officers (CWO), who are directly accessed from the senior enlisted grades (E-6 to E-9 and W-2 to W-5 for LDO; E-7 to E-9 for CWO), traditional unrestricted line officers are required to possess at least a bachelor's degree and complete some type of formal pre-commissioning accession program, such as: United States Naval Academy (USNA) Naval Reserve Officer Training Corps (NROTC), which also includes the Seaman to Admiral-21 Program Officer Candidate School (OCS). Some senior URL officers in current service as naval aviators or naval flight officers were also commissioned via the since disestablished Aviation Officer Candidate School (AOCS), either under traditional AOCS or via its former subset Aviation Reserve Officer Candidate (AVROC) or Naval Aviation Cadet (NAVCAD) programs. AOCS was merged into the current Officer Candidate School in the late 1990s. History The United States Navy takes most of its traditions, customs and organizational structure from that of the Royal Navy of Great Britain. Based on the Royal Navy model, there were originally two kinds of officers on a naval ship of the line: the commanding officers and their lieutenants, who were gentlemen and commanded the ship; and the warrant officers, who were technical specialists who ran important tasks. In the nineteenth century, with the introduction of steam power, a third group of officers emerged, engineers, who ran the steam plant. As technology developed, the engineers were requesting more rights, including command. This system evolved in similar fashion in the Continental Navy during the American Revolutionary War and in the successor United States Navy into the nineteenth century. Eventually, this dispute led the Department of the Navy to abolish the differences between the groups, amalgamating them into Unrestricted Line Officers in 1899. This fact can lead to confusion with non-American naval personnel, lacking the division between the two groups.