Because Lord Caitanya is an ocean of mercy, He is addressed as mahaprabhu, whereas Nityananda and Advaita, being two great personalities who assist Lord Caitanya, are addressed as prabhu
Personal associates like Gadādhara, Svarūpa Dāmodara, Rāmānanda Rāya, and the Six Gosvāmīs (headed by Rūpa Gosvāmī) are all situated in their respective transcendental humors. Thus the Lord submits to various positions in various transcendental mellows.
In verses 296 through 301 the emotional devoted service of Śrī Nityānanda, Śrī Advaita Prabhu and others has been fully described. Describing such individual service, the Gaura-gaṇoddeśa-dīpikā (11–16) declares that although Lord Caitanya Mahāprabhu appeared as a devotee, He is none other than the son of Nanda Mahārāja. Similarly, although Śrī Nityānanda Prabhu appeared as Lord Caitanya's assistant, He is none other than Baladeva, the carrier of the plow. Advaita Ācārya is the incarnation of Sadāśiva from the spiritual world. All the devotees headed by Śrīvāsa Ṭhākura are His marginal energy, whereas the devotees headed by Gadādhara Paṇḍita are manifestations of His internal potency.
Śrī Caitanya Mahāprabhu, Advaita Prabhu and Nityānanda Prabhu all belong to the viṣṇu-tattva category. Because Lord Caitanya is an ocean of mercy, He is addressed as mahāprabhu, whereas Nityānanda and Advaita, being two great personalities who assist Lord Caitanya, are addressed as prabhu. Thus there are two prabhus and one mahāprabhu. Gadādhara Gosvāmī is a representative of a perfect brāhmaṇa spiritual master. Śrīvāsa Ṭhākura represents a perfect brāhmaṇa devotee. These five are known as the Pañca-tattva.