A call-sign consists of a set of letters and numbers, assigned by the authority. For example VU2MSY (I was holding this call-sign for the last 10 years till I upgraded my licence with a new call-sign: VU2MUE) is a call-sign which belongs to a particular ham in India. No other hams anywhere in the world can have this call-sign! Similarly, 4S7VK is a call-sign which belongs to a ham from Sri Lanka. In a call-sign, the letters before the number is called the prefix. So, there might be a ham with the call-sign 4S7MSY in Sri Lanka. Similarly, a call-sign like VU2VK may exist in India! Each country has its own prefix. The knowledge of prefix helps us identifying the country of origin of a particular amateur radio transmission. Call-sign of all the ham radio operators are available in the International Call-book.
When a ham radio station is set up on board a ship, the call-sign should be followed the suffix /MM. So, VU2MSY would become VU2MSY/MM (maritime mobile) when it operates from a ship. If it operates from a vehicle, then the call-sign becomes VU2MSY/M (mobile).
Address from which the International callbook is available:
P.O. Box 2013, Lakewood, NJ 08701 USA