CC Ādi 5.10
‘śeṣa’-rūpe kare kṛṣṇera vividha sevana
He executes the orders of Lord Kṛṣṇa in the work of creation, and in the form of Lord Śeṣa He serves Kṛṣṇa in various ways.
According to expert opinion, Balarāma, as the chief of the original quadruple forms, is also the original Saṅkarṣaṇa. Balarāma, the first expansion of Kṛṣṇa, expands Himself in five forms: (1) Mahā-saṅkarṣaṇa, (2) Kāraṇābdhiśāyī, (3) Garbhodakaśāyī, (4) Kṣīrodakaśāyī, and (5) Śeṣa. These five plenary portions are responsible for both the spiritual and material cosmic manifestations. In these five forms Lord Balarāma assists Lord Kṛṣṇa in His activities. The first four of these forms are responsible for the cosmic manifestations, whereas Śeṣa is responsible for personal service to the Lord. Śeṣa is called Ananta, or unlimited, because He assists the Personality of Godhead in His unlimited expansions by performing an unlimited variety of services. Śrī Balarāma is the servitor Godhead who serves Lord Kṛṣṇa in all affairs of existence and knowledge. Lord Nityānanda Prabhu, who is the same servitor Godhead, Balarāma, performs the same service to Lord Gaurāṅga by constant association.