Here's why:

* Horizon Differentiation: This refers to the development of distinct layers (horizons) within a soil profile. These layers form due to different processes acting on the soil.

* Illuviation: This is the process where dissolved or suspended materials are transported (translocated) downwards within the soil profile and deposited in a lower horizon. This usually involves the movement of clay, iron, aluminum, and organic matter from the topsoil (A horizon) to the subsoil (B horizon).

* Translocation: This is the general term for the movement of soil materials from one horizon to another. Illuviation is a specific type of translocation where materials move downwards.

In contrast, eluviation is the process where materials are *removed* from a horizon, typically the A horizon, due to leaching. This is the opposite of illuviation.

Therefore, illuviation is a key process in horizon differentiation, specifically in the formation of the B horizon, where the translocated materials accumulate.