Оставлю здесь, чтобы скидывать ссылку при необходимости объяснить разницу между layers и tiers. The basic idea for the layered style is simple: components are organized in a layered fashion where a component at layer Lj can make a downcall to a component at a lower-level layer Li (with i < j) and generally expects a response. Only in exceptional cases will an upcall be made to a higher-level component. …. Many distributed applications are divided into three layers: (1) a user-interface layer, (2) a processing layer, and (3) a data layer. One approach for organizing clients and servers is then to distribute these layers across different machines…. As a first step, we make a distinction between only two kinds of machines: client machines and server machines, leading to what is also referred to as a (physically) two-tiered architecture. Источник: M. van Steen and A.S. Tanenbaum, Distributed Systems, 4th ed., distributed-systems.net, 2023. — Layers - логическая граница, tiers - физическая.