A posthumous title is the title given after the demise of the Emperor and gengo (era name) does not necessarily become his posthumous title from an institutional standpoint (after Meiji period the custom was just followed and nothing is prescribed for the posthumous title in Kyu Koshitsu-rei (the Old Imperial Families' Act) and so on).
追号は天皇が崩御した後に贈られる称号であり、制度上は必ずしも元号が追号になるわけではない（明治以降は慣例を踏襲しただけであり、旧皇室令等でも規定されていない）。