Bhagavad Gita · Raja Vidya Raja Guhya YogaChapter 9, Shloka 15 (9.15)

Three forms of worship (Shankara).

• ekatvena ekam eva paraṃ brahma iti paramārtha-darśanena — through the ultimate vision "there is only the one supreme Brahman";
• pṛthaktvena āditya-candrādi-bhedena sa eva bhagavān viṣṇuḥ avasthitaḥ — as distinct: "the same Vishnu is present in different forms, as the sun, the moon and so on";
• bahudhā avasthitaḥ sa eva bhagavān sarvatomukhaḥ viśva-rūpaḥ — as the one of many forms, the universal form facing everywhere.

And a question: yadi bahubhiḥ prakāraiḥ upāsate, kathaṃ tvām eva upāsate? — if they worship in many ways, how is it you alone they worship? — the answer is 9.16–9.19.

Acharya Ramanuja — one universal body. Bahudhā pṛthaktvena jagad-ākāreṇa viśvatomukhaṃ viśva-prakāram avasthitaṃ mām ekatvena upāsate — they worship me as one, though I exist separately in many ways in the form of the world; bhagavān vāsudeva eva … sūkṣma-cid-acid-vastu-śarīraḥ san … "sthūla-cid-acid-vastu-śarīraḥ syām" iti saṃkalpya … vicitra-jagac-charīraḥ avatiṣṭhate — Vasudeva, whose body is the subtle conscious and non-conscious, willed "may the gross world be my body", and exists in the form of the wondrous world-body.

The difference: Shankara — three separate forms of worship (non-dual, as distinct, as the universal form); Ramanuja — a single worship: the many forms in the world are the body of the one Lord.