The Will and Testament of ‘Abdu’l-Baha is the charter for the World Order of Baha’u’llah.

...the Will and Testament of ‘Abdu’l-Baha, written by His own hand during a period in which He was under great personal danger, is the charter for the World Order of Baha’u’llah. It is a document that had both immediate and future application. Shoghi Effendi wrote of it: ‘“We must trust to time, and the guidance of God’s Universal House of Justice, to obtain a clearer and fuller understanding of its provisions and implications.’’ It gave immediate authority for the development of the administrative institutions of the Faith. This document, ‘‘unique in the annals of the world’s religious systems,’’ assured the continuing unity and integrity of the Faith through the appointment of Shoghi Effendi, eldest grandson of ‘Abdu’l-Baha, as Guardian of the Faith of Baha’u’llah. It described how The Universal House of Justice ordained by Baha’u’llah would be elected and the relationship this supreme institution would have to the Guardianship and to the secondary Houses of Justice (National Spiritual Assemblies). It called for the appointment and outlined the duties of the Hands of the Cause of God, stressed the supreme authority of the Kitab-i-Aqdas, and warned of the danger and evil of Covenant-breaking. It made teaching the Cause the chief cornerstone of faith.

(From Strength to Strength, The First Half Century of the Formative Age of the Baha’i Era by Eunice Braun)