uttamaḥ puruṣas tv anyaḥ
yo loka-trayam āviśya
bibharty avyaya īśvaraḥ
uttamaḥ — the supreme, highest, best; puruṣaḥ — person, personality; tu — but; anyaḥ — another; parama-ātmā — the Supreme Soul; iti — thus; udāhṛtaḥ — is described; yaḥ — who; loka trayam—the three divisions of the world (universe); āviśya — enters; bibharti — maintains, sustains; avyayaḥ — eternal; īśvaraḥ — the Lord.
"Other than these two types of living beings, there is another who is described as the Supreme Soul, Paramatma, the imperishable transcendental Lord who has Himself entered the three worlds in His eternal form and is maintaining all living beings."
yasmāt kṣaram atīto ’ham
akṣarād api cottamaḥ
ato ’smi loke vede ca
yasmāt — because; kṣaram —to the perishable beings; atītaḥ — transcendental; aham — I am; akṣarāt — the imperishable beings; api — also; ca — and; uttamaḥ — superior; ataḥ — therefore; asmi — I am; loke — in the world; vede — in the Vedic literature; ca — and; prathitaḥ — celebrated; puruṣa-uttamaḥ — as the Supreme Person.
"Because I am transcendental to the perishable beings, and am also superior to My infallible eternal associates, My glories are sung throughout the universe and in the Vedas as that Supreme Person— Puruṣottamaḥ."
BG 15.17 Lecture Notes, references
Loke=worlds (material & spiritual) or universe consisting of the 14 planetary systems, and the 3 types refers to the inanimate (achetna) elemental world [elements-mahat tattva]; the combination of inanimate and animate world (Buddha-chetna); and the para—transcendental category—the unlimited spiritual world.
The first 2 categories are of jivas in bondage and the last is the world of jivas who are liberated and situated in the proper soul consciousness in the spiritual worlds—akṣarāt api ca uttamaḥ (the superior beings who never fall down from their position).
Param-atma –the Supreme Soul is different from the temporary but imperishable souls as well as the individual immortal souls, and sustains all the living entities and the worlds. The temporary but imperishable refers to the souls trapped in illusion, and the individual immortal souls are the liberated ones.
The Supreme Soul maintains all living entities in all creation as one undivided reality, but each having their own individuality. The finite entities cannot become the Supreme Soul because the Paramatma is always param—superior and above them all. [While the finite eternal living entity souls are part and parcel of the Supreme Soul, the Supreme Soul is infinite, and separate and distinct from each of these. The individual soul can never become the Supreme Person, even though impersonalist philosophies wrongly claim this on the basis that the individual soul is part and parcel of the Supreme.]
Chāndogya Upaniṣad (8.12.3): tāvad eṣa samprasādo ’smāc charīrāt samutthāya paraṁ jyoti-rūpaṁ sampadya svena rūpeṇābhiniṣpadyate sa uttamaḥ puruṣaḥ. “The Supersoul coming out of the body enters the impersonal brahma-jyotir; then in His form He remains in His spiritual identity. That Supreme is called the Supreme Personality.” [ACBSP quoted in Vaniquotes]
tata eva ca vistāraṁ
brahma sampadyate tadā
yadā — when; bhūta — of living entities; pṛthak-bhāvam — separated identities; eka-stham — situated in one; anupaśyati — one tries to see through authority; tataḥ eva — thereafter; ca — also; vistāram — the expansion; brahma — the Absolute; sampadyate — he attains; tadā — at that time.
When a sensible man ceases to see different identities due to different material bodies and he sees how beings are expanded everywhere, he attains to the Brahman conception.
BG 15.18 Lecture Notes, references
Both the Muṇḍaka Upaniṣad (3.1.2) and Śvetāśvatara Upaniṣad (4.7) confirm this:
samāne vṛkṣe puruṣo nimagno
'nīśayā śocati muhyamānaḥ
juṣṭaṁ yadā paśyaty anyam īśam
asya mahimānam iti vīta-śokaḥ
"Although the two birds are in the same tree, the eating bird is fully engrossed with anxiety and moroseness as the enjoyer of the fruits of the tree. But if in some way or other he turns his face to his friend who is the Lord and knows His glories—at once the suffering bird becomes free from all anxieties." --[ACBSP, quoted in Vaniquotes]
Additional Study reference--Whole of BG Chapter 8, specially 8.4-5; 8.14-15: [by remembering Krishna always, the nittya baddha (eternally bound) can attain to Him.
Srimad Bhagavad Gita, The Hidden Treasure of the Sweet Absolute - Srila Bhakti Rakshak Sridhar Swami, (SCSMath) [Downloadable PDF];  -(2nd edition 2006)
Bhagavad Gita As It Is - Srila A.C. Bhaktivedanta Swami Prabhupada, Macmillan 1972.