When breaking through to the Nascent Soul Realm, a cultivator shatters his Golden Core and condenses the pure energy inside it into a Nascent Soul.
Whereas in the Soul Transformation Realm, the Nascent Soul undergoes a transformation, evolving into a Divine Soul, which is often described as the cultivator's true self or Dao Soul.
Those who reached this level are considered half-immortals because their souls were powerful enough to leave their bodies.
As for the reason why they are called half-immortals, it is because these Souls could create their own bodies using some special resources.
Of course, it is a very costly process that involves not just money but risk.
Although the names of the cultivation realms for the Humans and the Undead Beings might be different, the essence was the same, just the element was different.
Like how their Nascent Souls turn into Divine Souls, the Undead Beings would transform their Nether Souls into Abyssal Souls.
