Basically, there are two schools of Indian scriptures, which are the Agama and the Nigama.
The Agama scriptures feature revelations while the Nigama scriptures feature the traditions. Tantra is an Agama, and hence it’s referred to as Sruti Shakhavisesah.
In total, there are 18 Agamas, which are also known as Shiva Tantras; they’re ritualistic in nature. There are 3 main Nigamas, which include Dakshina, Vama, and Madhyama. These traditions represent the 3 powers of Shiva characterized by the 3 Gunas: Sattva, Rajas, and Tamas, respectively.