For years, SAP developers working on S/4HANA faced a dilemma: how do you modernize custom code without disrupting a system that runs critical business processes? The answer has evolved significantly — and the latest Clean Core Level Concept finally provides a pragmatic, structured answer.
From the 3-Tier Model to Clean Core Levels
The original 3-Tier Development Model organized ABAP extensibility into three layers: Tier 1 for cloud-ready ABAP Cloud development, Tier 2 for wrapper-based compatibility, and Tier 3 for classic legacy ABAP. While this provided useful guardrails, it often forced developers into architectural choices that were technically expensive or simply impractical for transactional-heavy logic.
The new Clean Core Level Concept replaces this rigid structure with a more nuanced maturity model — Levels A, B, C, and D — each reflecting a different degree of cloud readiness and adherence to clean core principles.
Understanding the Four Levels
Level A: The Gold Standard
Level A represents the highest degree of clean core compliance. Development at this level uses only publicly released SAP APIs, follows ABAP Cloud rules enforced by the ABAP Language Version “ABAP for Cloud Development,” and is fully upgrade-safe. Extensions built at Level A can be deployed side-by-side on SAP BTP or on-stack in S/4HANA Cloud Public Edition without modification.
Level B: Controlled On-Stack Extension
Level B allows on-stack development in S/4HANA Private Cloud or on-premise, using released APIs and selected classic ABAP constructs that are covered by SAPs upgrade compatibility promise. This level is ideal for complex transactional scenarios where proximity to data is critical, but where developers still want to stay within a well-governed extensibility boundary.
Level C: Governed Classic Extension
Level C represents a transitional zone — classic ABAP extensions that are documented, reviewed, and monitored for upgrade risk. These are not yet clean core compliant, but they are tracked and managed. Organizations pursuing a clean core journey often start here, using ATC (ABAP Test Cockpit) checks to identify and remediate Level C code over time.
Level D: Unmanaged Legacy Code
Level D is the technical debt zone — classic ABAP modifications, direct table accesses, and undocumented customizations that create upgrade risk and hamper cloud adoption. The primary goal of any clean core initiative is to eliminate or migrate Level D code to higher levels.
Why This Framework Matters for Your S/4HANA Journey
The Clean Core Level Concept gives organizations a concrete roadmap. Rather than treating clean core as a binary pass/fail state, it acknowledges that large enterprise landscapes contain a spectrum of custom code — and that a measured, prioritized migration path is far more realistic than a “rip and replace” approach.
For ABAP development teams, understanding which level your custom code sits at — and what it would take to move it up — is the foundation of a successful S/4HANA cloud transformation strategy. The ATC check framework makes this classification automatable, enabling continuous governance rather than a one-time audit.
At Clean Core ABAP, we help organizations assess their current extensibility posture, classify custom code across these levels, and define a targeted migration roadmap that balances technical ambition with operational reality.