Only in the Marine Corps are master sergeants required to be addressed by their full rank. In the Marine Corps, master sergeants may be referred to by the nickname of "Top". This usage is an informal one, however, and would not be used in an official or formal setting. Use of this nickname by Marines of subordinate rank is at the rank holder's discretion. In the U.S. Armed Forces, all master sergeants (Army, Air Force, and Marine Corps) are senior non-commissioned officers (i.e., pay grades E-7 through E-9). However, in the U.S. Marine Corps, the non-commissioned officer ranks of staff sergeant and above, are classified as Staff Non-Commissioned Officers (SNCOs), a classification that is unique in U.S. usage to the USMC.