After reunification the German Navy kept the system, codes for ships from the former forces of East Germany (Volksmarine) were added.
This letter is usually spoken and sometimes written according to the NATO Phonetic Alphabet, e.g. class 206 alpha for the upgraded Type 206 submarine.
Classes acquired from foreign nation or from World War II were given numbers near the end of their block.
Especially for those that were soon sold afterwards to foreign navies, this process did not strictly follow the previous rules.
But the vast majority was categorized in the previously unused 600-699 range, but even here the same Type code is often used more than once.