A Super Heavy System Defense Vessel is the classification given to the largest vessels purpose-built to the needs of system defense of a crystal sphere. Whether that be interdiction, eleemosynary aid or defense against hostile forces, these ships provide the final word on the subject. At this time, the Tinris World Navy only employs and maintains the Citadel classof superheavy system defense vessels. Additionally, this remains by custom one of the few types of vessels within the World Navy which may be, and often are called 'boats'. The reason for this distinction would seem to be lost to history.