“The Nichiren Sect” in Nanjō [Bunyu]’s Short History of the Twelve Japanese Buddhist Sects contains the following remarks on the Kaidan.

The Kaidan, or “place for receiving instruction in śīla or moral precepts,” of the original doctrine is explained as follows: To keep the śīla is the most important matter of all the divisions of Buddha’s doctrine, whether of the great or the small vehicle of the true or of the temporary. Therefore there is in the original doctrine the first true śīla which is held by Buddha permanently. The Kaidan is the Bodhi-manda (dō jō) or “place for the way,” where the ceremony to receive instruction in the śīla is to be accomplished. The place is now mentioned instead of the law which is to be observed there.

The substance of this śīla is the title of the five characters Myō-hō-ren-ge-kyō. One who believes in this title and observes it, is said to be the holder of the excellent śīla of the original doctrine. The place where he keeps and holds it, is the Pure Land of the Calm Light (Jak-kō-jō-do), i.e. the Kaidan. (pp. 145-146)

The Kaidan of the Nichiren Sect is understood to be a practical Kaidan (ji no kaidan) and not a theoretical Kaidan (ri no kaidan) as found in the Tendai Sect. The former expresses the meaning of Hommon or the Original Teaching, the latter that of Shaku Mon or the Trace Teaching. As stated above, the Kaidan of the Nichiren Sect is the Dōjō or “place for the way” (the term “way” meaning here ”the way leading to enlightenment”) where the believer in the Hoke-kyō recites the Title and meditates on it. Upon attaining enlightenment Śākyamuni once sat in such a place, and every Bodhisattva sits down on such a seat before he becomes Buddha. It is said to be the navel or center of the earth, the terrace of enlightenment raising itself on the bodhi-site, and to be diamond-like; consequently it is also called the “diamond throne.”