Bhagavad Gita Chapter 15, Verse 17

उत्तमः पुरुषस्त्वन्यः परमात्मेत्युदाहृतः | यो लोकत्रयमाविश्य बिभर्त्यव्यय ईश्वरः ||

uttamaḥ puruṣas tv anyaḥ paramātmety udāhṛtaḥ yo loka-trayam āviśya bibharty avyaya īśvaraḥ

But distinct is the Supreme Person, called the highest Self, indestructible and Lord, who pervades the three worlds and sustains them.

Krishna reveals the third category—Purushottama: 'Uttamaḥ puruṣas tv anyaḥ paramātmety udāhṛtaḥ'—but distinct is the Supreme Person, called the highest Self. This is different from the perishable (all material beings) and different from the imperishable (the individual soul). Purushottama is the third—the Supreme Person who is both immanent and transcendent. 'Yo loka-trayam āviśya bibharty avyaya īśvaraḥ'—who pervades the three worlds and sustains them, indestructible and Lord. Purushottama permeates all three worlds (heaven, earth, and the netherworld) and sustains them. He's 'avyayaḥ'—indestructible, and 'īśvaraḥ'—the Lord. This is the Supreme Person—not just the individual soul, but the Supreme Self who is the source and sustainer of everything. Understanding Purushottama is the key: He transcends both the perishable (material) and the imperishable (individual soul). He's the highest—the source from which everything comes and the goal to which everything returns.