The letters are linked to the 28 lunar mansions (manazil al-qamar), the planets, the elements (fire, water, air, earth), and the stages of creation. For example, Alif represents the First Intellect (al-Aql al-Awwal), Ba represents the creation of the cosmos, etc.
: Associated with the "Sir" (secret) of the soul and is often linked to the Prophet Muhammad as the "Heart of the Quran". ilm al-huruf pdf