The Four Worlds are widely correlated with the four letters of the Tetragrammaton, and Assiah, the fourth and lowest, is assigned the final He, read in this scheme as the completion of the name, the point where the pattern handed down from Yetzirah becomes fully dense, material and particular. The correspondence treats the whole descent from Atziluth to Assiah as the successive spelling out of a single divine name across four worlds of increasing solidity, ending where the name itself ends.
Kabbalistic teaching holds that Assiah, like each of the Four Worlds, contains a complete Tree of Life within it, its own ten sefirot, so that the physical universe and the realm of everyday human deed is not simply the bottom of a list but a whole structure in miniature, receiving the formative pattern handed down from Yetzirah above it. Because Assiah is the last world, this nested tree is also the one most directly identified with material existence, and some sources hold it to contain a further, coarser layer, the world of shells or kelipot, beneath even its own tree.
Assiah corresponds in the soul scheme to Nefesh, the level bound most closely to the body, placing basic instinct and appetite at the same rung as the world of matter and action, the pairing Jewish mourning custom draws on when it describes Nefesh as lingering near the grave.
Sources The ZoharTraditionally attributed to Moses de Leon with Encyclopaedia Judaica, 2nd editionMichael Berenbaum and Fred Skolnik (eds.)