Bhagavad Gita · Akshara Brahma YogaChapter 8, Shloka 20 (8.20)

Two unmanifests. The first — the seed of creation and dissolution (which changes); the second — eternal, which never changes.

Shankaracharya. Tu-śabdaḥ akṣarasya vivakṣitasya avyaktāt vailakṣaṇya-viśeṣaṇārthaḥ — "tu" (but) is there to show that the akshara is different from the first unmanifest; bhāvaḥ akṣarākhyaṃ paraṃ brahma — this reality is the supreme Brahman called akshara; the first unmanifest is bhūta-grāma-bīja-bhūtāt avidyā-lakṣaṇāt — the seed of the multitude of beings, of the nature of ignorance; sanātanaḥ … sarveṣu bhūteṣu brahmādiṣu naśyatsu na vinaśyati.

Acharya Ramanuja — the pure self. Tasmād avyaktād acetana-prakṛti-rūpāt puruṣārthatayā para utkṛṣṭo bhāvaḥ anyo jñānaikākāratayā tasmād visajātīyaḥ — higher than that unmanifest, which is non-conscious prakriti, as a goal of human life, and of a different kind from it because its sole form is knowledge; avyaktaḥ kenacit pramāṇena na vyajyata iti … sva-saṃvedyāsādhāraṇākāraḥ — it is not revealed by any means of knowledge, it is known only by one's own experience; tatra tatra sthito api na vinaśyati — though dwelling in beings, it does not perish.

The difference: Shankara — the supreme Brahman; Ramanuja — the pure individual self freed from prakriti (the goal of the seeker of kaivalya).