Lord Śrī Kṛṣṇa is the one Absolute Supreme Personality of Godhead, but He has expanded Himself into His multiplenary portions by His inconceivable energy. The conception of duality is due to ignorance of His inconceivable energy. In the Bhagavad-gītā (BG 9.11) the Lord says that only the foolish take Him to be a mere human being. Such foolish men are not aware of His inconceivable energies. By His inconceivable energy He is present in everyone's heart, as the sun is present before everyone all over the world. The Paramātmā feature of the Lord is an expansion of His plenary portions. He expands Himself as Paramātmā in everyone's heart by His inconceivable energy, and He also expands Himself as the glowing effulgence of brahmajyoti by expansion of His personal glow. It is stated in the Brahma-saṁhitā that the brahmajyoti is His personal glow. Therefore, there is no difference between Him and His personal glow, brahmajyoti, or His plenary portions as Paramātmā. Less intelligent persons who are not aware of this fact consider brahmajyoti and Paramātmā to be different from Śrī Kṛṣṇa. This misconception of duality is completely removed from the mind of Bhīṣmadeva, and he is now satisfied that it is Lord Śrī Kṛṣṇa only who is all in all in everything. This enlightenment is attained by the great mahātmās or devotees, as it is stated in Bhagavad-gītā (BG 7.19) that Vāsudeva is all in all in everything and that there is no existence of anything without Vāsudeva. Vāsudeva, or Lord Śrī Kṛṣṇa, is the original Supreme Person, as now confirmed by a mahājana, and therefore both the neophytes and the pure devotees must try to follow in his footsteps. That is the way of the devotional line.
The worshipable object of Bhīṣmadeva is Lord Śrī Kṛṣṇa as Pārtha-sārathi, and that of the gopīs is the same Kṛṣṇa in Vṛndāvana as the most attractive Śyāmasundara. Sometimes less intelligent scholars make a mistake and think that the Kṛṣṇa of Vṛndāvana and that of the Battle of Kurukṣetra are different personalities. But for Bhīṣmadeva this misconception is completely removed. Even the impersonalist's object of destination is Kṛṣṇa as the impersonal jyoti, and the yogī's destination of Paramātmā is also Kṛṣṇa. Kṛṣṇa is both brahmajyoti and localized Paramātmā, but in brahmajyoti or Paramātmā there is no Kṛṣṇa or sweet relations with Kṛṣṇa. In His personal feature Kṛṣṇa is both Pārtha-sārathi and Śyāmasundara of Vṛndāvana, but in His impersonal feature He is neither in the brahmajyoti nor in the Paramātmā. Great mahātmās like Bhīṣmadeva realize all these different features of Lord Śrī Kṛṣṇa, and therefore they worship Lord Kṛṣṇa, knowing Him as the origin of all features.