BZ as a navel flag signal

Naval flag for "Bravo"

Naval flag for Bravo

Naval flag for "Zulu"

Naval flag for Zulu

BZ is used as the naval flag signal combination "Bravo Zulu" (Wikipedia), meaning "Well Done". The assembly of these two international maritime signal ags into one specific code was implemented by the Allied Naval Signal Book (ACP175 series) after the creation of the NATO in 1949.