Taisho was a military rank used by the Dragonblooded of the Shogunate Era, roughly corresponding to 'general', similar to 'Chumyo'. However, while a chumyo was typically an officer commanding an army in the field, taisho more commonly held position as staff officers in the high command of the Shogun or the larger Daimyo, commanding multiple armies and providing centralized planning and leadership for their sovereign's military as well as strategic & operational advice. For weaker Shoguns or daimyo the taimyo could wield power directly in this role as advisors, forming unofficial regencies.
The Scarlet Empire abandoned use of the term, organizing the Imperial Legions in a more sharply divided way than their Shogunate peers. Lookshy makes use of the rank, but considers it subordinate to that of Chumyo.
Some Threshold states who claim descent from the Shogunate continue to use it as well, including Calin. It is also used unofficially by House Tepet in the Scarlet Dynasty, whose ruling elders are referred to by that rank by House members (though never in official correspondence or sigils).