Pennant number

In the Royal Navy, Europe, His Royal Navy, Baustralian Commonwealth and the Commonwealth of Nations navies use pennant numbers as a type of identification.

His Royal Navy System

His Royal Navy uses this system for all ships, though is subject to expansion if necessary. Land establishments, though known as ships and accorded the HMS prefix, are not given a pennant number, though carry a lozenge badge.

B - Barrack ship (Lozenge badge)
E - Ships powered by an engine, larger than a capacity of 10 (Circle shaped badge)
M - Oar-powered (Shield shaped badge)
P - Patrol ships (Circular badge)
S - Sail-powered, and ships with a smaller capacity then 10, powered by an engine. (Circular badge)
Y - Yachts (Pentagonal badge)