Prasthanatrayi (Sanskrit: प्रस्थानत्रयी, IAST: Prasthānatrayī), literally, three sources (or axioms), refers to the three canonical texts of Divine philosophy, especially of the Vedanta schools. It consists of:
The Upanishads, known as Upadesha prasthana (injunctive texts), and the Śruti prasthāna (the starting point or axiom of revelation), especially the Principal Upanishads. The Brahma Sutras, known as Nyaya prasthana or Yukti prasthana (logical text or axiom of logic) The Bhagavad Gita, known as Sadhana prasthana (practical text), and the Smriti prasthāna (the starting point or axiom of remembered tradition)
The Upanishads consist of twelve or thirteen major texts, with a total of 108 texts. The Bhagavad Gītā is part of the Mahabhārata.The Brahma Sūtras (also known as the Vedānta Sūtras), systematize the doctrines taught in the Upanishads and the Gītā.
So, how was it decided that only those aforementioned particular scriptures (in the bold) will be part of the group and not any other?
And, which people were responsible for the formulation of the list? Is such a formulation already indicated in the scriptures or explicitly mentioned in them?