Absolute Free Architecture-Specialist Exam Practice for Comprehensive Preparation 

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

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. 

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.

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