DDH, DDG, FFH, CVN, SSBN, SSK, etc are not abbreviations or acronyms per se (i.e. the letters don't necessarily stand for specific words in a specific order). They are simply codes used by NATO to designate different types of vessels. I'm guessing the system came about in the early days of computers when there was a requirement to keep the number of characters used to a minimum. Although some letters do seem to correspond to specific words (in English), that is not always the case. For example, I don't think a "YAG" is a "yard auxiliary, guided missile."