Sri Vidya is a Shakta sect which believes that the goddess Lalita Tripurasundari is supreme. Now the Varivasya Rahasya is a work by the 18th-century Sri Vidya philosopher Bhaskararaya explaining the meaning of the Panchadasi mantra, the core mantra of the Sri Vidya sect described in this web page. In this excerpt from the Varivasya Rahasya, Bhaskararaya explains the significance of certain sounds in the Panchadasi mantra by quoting the following verse from a Shakta Agama called the Yogini Hridaya:
Rudra, Ishvara, and Sadashiva are indicated by the three Bindus. Shanti, Shakti and Shambhu are signified by the three Nadas.
Now as I discuss in this question, in the worldview of the Shaiva Agamas, the hierarchy of gods from lowest to highest is Brahma, Vishnu, Rudra, Maheshwara, and Sadashiva. So I assume the "Rudra, Ishvara, and Sadashiva" part is a reference to the last three names on that list.
But my question is, what form of Shiva is being referred to by the name "Shambhu"? Does the worldview of the Shakta Agamas add an additional being named Shambhu to the five gods delineated in the Shaiva Agamas?
Does anyone know if the Yogini Hridaya has more information on Shambhu? I don't have an English translation of the Yogini Hridaya, but here is a Hindi translation.