SB 1.3 - The paribhāṣa-sūtra of Bhāgavatam
Now how can we establish Kṛṣṇa is the complete form of God on the basis of one statement when there are apparently other statements that refute this. For instance SB 10.1.2 says to describe the wonderful activities of the expansion of Lord Viṣṇu, who appeared in the Yadu dynasty (tatra amśenāvatīrṇasya viṣṇor). SB 10.2.41 says the Supreme Lord Himself as an expansion is now within the womb of Devakī (amśena sākṣād bhagavān).
In the beginning of Bhāgavatam, this chapter concerning the appearance of the Lord is called a sūtra, since it threads together statements concerning all avatāras. And in this chapter this verse 1.3.28 is a paribhāṣa-sūtra, which supplies a general definition for the whole work. A paribhāṣa-sūtra regulates something that is not otherwise regulated; it creates a rule or restriction where none existed previously. A book’s paribhāṣa-sūtra acts as a lens thru which the entire book can be seen. It dictates how all succeeding points in the book should be interpreted. In a lengthy Sanskrit work containing many indirect meanings, the author’s true and consistent intention can be understood thru the book’s paribhāṣa-sūtra. Both Jīva Gosvāmī and Śrīdhara Svāmī accept this verse as the paribhāṣa-sūtra of Bhāgavatam. Thus wherever avatāras are described in the Bhāgavatam, others should be knows as expansions of the puruṣāvatāras, but Kṛṣṇa should be known as svayam bhagavān. Thus the above objection that Kṛṣṇa is an aṁśa is addressed.