Shoretsu means the superiority of the Essential Teachings of the Lotus Sutra (Chapters 15 - 28) over the Theoretical Teachings (Chapters 1-14). Itchi is the equality of all the Chapters of the Lotus Sutra, the Theoretical Teachings and the Essential Teachings. There were many debates on this subject since the time of Nichiren. All schools consider the Daimoku first and foremost but regarding the auxilliary teachings, the following is true...
The Shoretsu Schools are: The Soka Gakkai; the Nichiren Shoshu; the Honmon Butsuryu Shu (the Eight Chapter School, Chapters 15 - 22); the Kempon Hokke (the One Chapter and Two Half Chapters School, Chapter 16, the Latter Half of Chapter 15 and First Half of Chapter 17). and us. We all take the Essential Teachings as superior to the Theoretical Teachings.
The Itchi Schools are: the Nichiren Shu; and the Tientai School. They embraces the doctrine of the Equality of the Theoretical and Essential sections of the Lotus Sutra.
Nichiren was a Shoretsu practitioner. It baffles me why the Nichiren Shu embraces the Itchi.
The Soka Gakkai, Nichiren Shu, and even the Honmon Butsuryu Shu are Shoretsu but they all assert that the Twenty-Eight Chapter Lotus Sutra has lost its power in Mappo
The Kempon Hokke, us, and as mentioned, Nichiren, are Shoretsu but we believe the Daimoku is the Entity of the Lotus Sutra. This is likened to the brain and spinal chord as the entity of a complex organism composed of various organ. Namu Myoho renge kyo is the brain and spinal cord of the 28 Chapter Lotus Sutra. A complex organism can not live without a brain and spinal chord (the master and most important organ). Similarly, the brain and spinal cord dies if the other organs are diseased or absent. The 28 Chapter Lotus Sutra is indispensable for attaining Buddhahood as the other major organs are indispensable for keeping alive the brain and spinal cord.
The major organs without which the organism dies is the heart and blood vessels, the liver, lungs, kidneys, digestive system, lymphatics, and skin. The minor organs such as the spleen, bladder, and genitals can be diseased or absent without the death of the organism. However, without these minor organs, the human being will be prone to disease, will experience discomfort, and will be less pleasurable. The major organs may be likened to the Essential Teachings of the Lotus Sutra and the the minor organs to the Theoretical Teachings.