Clean Core

A clean core means that SAP standard code is not modified and that extensions use only stable, documented interfaces: released APIs, released CDS views and extension points such as BAdIs. When an extension depends only on released objects, a new release is far less likely to break it.

Existing custom code usually departs from the principle in a few recurring ways: direct writes to SAP standard tables, use of objects without a release contract, modifications and implicit enhancements of standard code, and dependencies on objects that are deprecated in the target release.

SAP describes how far an extension follows the principle with a tier model and, more recently, with more granular clean core levels. Which classification applies depends on your release, so check the current SAP documentation for it.