Why a high-low fleet mix needs a middle tier

The three-tier Japanese fleet model places large Aegis ships at the top, Mogamis in lower-level roles, and 13DDX in the middle. The medium ship changes function with the threat environment. In high-threat operations it supplies escort numbers and backbone capability, while the lowest tier generally stays outside those scenarios. In moderate or lower-threat operations it supplies the leading combat capability, organises smaller units, and represents the state independently. It can then join larger formations as part of their escort backbone. This changing role explains how a middle tier contributes to a high-low mix. The US Navy's major combatants and Coast Guard coverage are judged to leave a similar need for a medium naval escort. The initial FF(X) batch is considered closer to the lower tier.