What is the difference between al-wahy (revelation) and at-tanzil (sending down) linguistically and doctrinally, and why were both terms used in the Holy Qur’an?

Linguistically, revelation (al-wahy) is hidden communication. It is also applied to writing, resurrection, inspiration, command, and signaling. Religiously, it is the communication of the divine law. Sometimes, it refers to the revealed scripture itself, which is the word of Allah sent down to the Prophet, peace and blessings be upon him. As for al-tanzil (sending down) linguistically, it is the verbal noun of nazala (to descend), and it is applied to the Qur’an itself. The fact that the Qur’an is a “sending down” (tanzil) from the Lord of the Worlds indicates that Allah is above His creation and that the Qur’an descended from Him. The secret behind preferring one expression over the other in certain contexts has not been clarified.