Side-by-Side Extension
What it is
An extension that runs as its own application on SAP BTP (in ABAP Cloud, CAP/Node.js or Java) and talks to S/4HANA only over released APIs and events. The core stays untouched; the extension has its own lifecycle.
Why it matters for Clean Core
It's the maximum-decoupling option: heavy custom processing, third-party integration, or anything you don't want tied to the S/4HANA release cadence belongs here. It keeps the digital core clean by definition.
Common pitfall
Going side-by-side for logic that is tightly transactional with core data — you pay a latency and consistency tax for the decoupling. Side-by-side is for loosely-coupled extension, not for re-implementing core transactions.