Final answer:
A flag officer or unit commander afloat is traditionally allowed to display their personal flag or command pennant from only one vessel at a time. This protocol signifies the officer's presence on a particular ship and maintains clear command identification within the fleet.
Step-by-step explanation:
The question concerns naval protocol and traditions related to the display of flags and command pennants aboard ships, which is a part of maritime history and customs. In military naval tradition, a flag officer or unit commander afloat is permitted to display their personal flag or command pennant to signify their presence aboard a vessel. This flag or pennant is known as a distinguishing mark of their command authority and status. However, according to naval customs, a flag officer or unit commander is limited to displaying their personal flag or pennant from only one vessel at a time. When the flag officer or commander moves to another ship, the flag or pennant is transferred to the new ship to indicate the presence of the high-ranking officer or authority.
This practice has a long history and is deeply rooted in naval tradition. For instance, historical records such as the 'Dictionary of vexillology' mention signs like the 'baucans', a red streamer used in the 13th century to signify a commitment to battle. Such traditions continue in modern times, although the specifics can vary based on the country's military regulations and maritime laws.
In conclusion, while a flag officer or unit commander might have multiple ships under their command, they may only display their personal flag or command pennant on one vessel at any given time. This protocol ensures clear identification of the vessel that the officer or commander is actually on at any moment, thus maintaining order and hierarchy within the naval fleet.