In Lore info:

Federation Naval Command is, as the name implies, the supreme command for the Federal Navy. A joint defense council comprised of the highest honoured admiralty of the Federal Navy is selected to be apart of this council, being referred to by an unofficial title of High admiral. This council was formerly commanded by Admiral of the Fleet, Lucas Vincent, but was replaced by Fleet Admiral Curtis Vantor.

In game info:

Federation Naval Command is a large, military based group with the intentions of emulating a practical structure and chain of command while also having an enjoyable time, all with the intent of supporting and uniting the Federation. Under FNC, we have 3 operational fleets, the 21st, 8th and 43rd Fleet. Each fleet is run by it's own admiral and is allowed to operate with a certain level of autonomy. While the entire Navy has a standardized ranking structure, how each fleet chooses to organize it's members is strictly up to them. An example could be how the 21st Fleet puts 4 PO3's into a 'wing," two wings are put into a Flotilla and three Flotillas are put into a Task Force. This specific way of organizing is unique to the 21st fleet and is not a requirement to follow.