Ukraine has adopted NATO’s standard system for numbering warships. This is the originally British pennant number system that most NATO countries use. By this system the number is preceded by a letter to identify the kind of ship (A = Auxiliary, F = Frigates or corvettes, P = Patrol vessels, M = Minelayers or mine hunters, L = Landing ships, etc.). That writes the Russian defense blog bmpd.ru with a link to ukrmilitary.com.
After the collapse of the Soviet Union, Ukraine made their own system of numbering warships whereby all numbers were preceded by the letter “U”. However, at a celebration in Odesa on 29 April 2018 it was noticed that some ships have changed numbers. Some patrol boats now show the letter “P”, and some auxiliary vessels have the letter “A”. The numbers themselves are unchanged, so only the letters are new.
This is a possible source of future confusion. The Ukrainian training vessels Chygyryn, Smila, and Nova Kakhovka have received the numbers A540, A541, and A542. Incidentally these are also the pennant numbers of the Danish royal yacht Dannebrog (A540), and the hydrographic surveying ships Birkholm (A541) and Fyrholm (A542).
NATO’s pennant number system was originally invented in the British Royal Navy in 1910. It was revised in 1948, and in 1954 it was adopted as a common system for the European NATO countries. However, it is not really a NATO standard. The United States, Canada, and some countries that became members after 1992 still use national systems.