The flagship is a mobile command post: a battleship-size ship that acts as a relay and command post in the Infonet, the network that carries orders to ships. It covers its own system and every system one jump away, and its coverage moves with it. Orders to ships in that coverage can enter the Infonet at the flagship. While jumping, it relays nothing.
A Shipyard builds it from heavy frames, shielding, a control unit, a field coil and thrusters. The heavy frames, shielding and field coil all take exotics, and shielding is made at a Foundry.
It has a small hold, and only an Anchorage's dock takes it. Unlike an uplink, which stays in its station, it carries its command post with it.