The separate category gives Japan enough battlecruiser tonnage to retain all four Kongos, with roughly 16,000 tons left for improvements. A 37,000-ton battleship category then opens a choice between reducing the Tosa design, adapting Kii ideas, or beginning an entirely new fast battleship. Four new ships would affect Japanese cruiser work and later capital-ship development, but they would not overturn the treaty ratio. These options give Japan visible scope for renewal and the prestige of continued construction. That sense of naval security matters politically because states that feel less threatened are less likely to interpret every rival move as existential.
Chapter · 52:27-56:58
Japan's Kongo force and new battleship choices
Japan could retain all four Kongos within a separate battlecruiser allowance while gaining room for four new battleships, changing both force planning and perceptions of security.