User blog comment:Dragonjet/KC3改/@comment-25567598-20150512035201/@comment-1820055-20150512130836

DDE and XBB are classifications found on game data, but have no ship incorporated to them as of now, but they exist in the ship-type list.

DDEs or "Escort Ship" (see Easter Eggs) is speculated to be released somewhere in the future, see Recent Updates.

XBBs or "Super Dreadnought" are really unknown, but still on the game's master data.