_Eng module is for

Isolated Business Logic (Actions) or Core Widgets (blocks), to manage complexity, compositio or to have its own lifecycle.

Logic to Synchronize data in CS's with an external system. Isolating this logic makes the CS completely system agnostic and it's easier to decouple or replace the external system.

A BL becomes a Calculation Engine if it performs complex calculations, (e.g. an invoice calculation engine or an insurance simulator). Engines are usually subject to versions.

Technical wrapper to expose an API to External consumers, keeping core services system agnostic and supporting multiple versions of the API.

Reusable Core Services with public entities, actions, and blocks.