Lesson 1 of 5
From tiers to levels
9 min
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Contoso Motor Co.'s system conversion from SAP ECC 6.0 EHP 8 to SAP S/4HANA Private Edition carried every custom object across, adapted where the simplifications required it. A month after the second mock conversion, Kenji, the ABAP development lead, is asked by Grace's program board a simple question: "How clean is our core?" He has two vocabularies to answer with, an older three-tier model and SAP's newer clean core levels. This lesson explains both and why SAP moved from one to the other.
Clean core in one paragraph
SAP's clean core strategy aims to keep SAP S/4HANA close to standard and upgrade-stable while still letting customers build what differentiates them. For extensions, that means building on interfaces SAP promises to keep stable, so that an upgrade does not break custom code and custom code does not block an upgrade. Extensions can run side-by-side on SAP BTP, loosely coupled, or on-stack in the S/4HANA system using the ABAP Cloud development model. SAP's current guidance is to choose per use case, with a BTP-first mindset.
The three-tier model
For on-stack development in SAP S/4HANA Private Edition and on-premise, SAP described a three-tier extensibility model, which you will still find in SAP Learning material and older project documents.
| Tier | What it is | Upgrade stability |
|---|---|---|
| Tier 1 | Cloud-ready development with ABAP Cloud: the restricted language version, released APIs and extension points only | Highest; SAP keeps released APIs stable |
| Tier 2 | Custom wrappers around SAP objects that have no released API yet, so Tier 1 code can call them | Contained; the risk sits in a small wrapper layer |
| Tier 3 | Classic ABAP with no restrictions: any SAP object, modifications, implicit enhancements | Lowest; any upgrade may affect it |
The earlier three-tier model for on-stack extensions in private cloud and on-premise systems.
The tiers described how code was built. They lumped very different things into Tier 3: a call to a well-documented BAPI and a direct UPDATE on an SAP table were both "classic". For a program like Contoso's, with years of ECC code, nearly everything landed in Tier 3, which told the board little about where the real upgrade risk sat.
The clean core levels
In August 2025, SAP announced that it had evolved the three-tier model into the clean core level concept, with four levels from A to D. Levels rate an extension by what it uses and therefore how upgrade-stable it is. Level A uses only publicly released, stable interfaces. Level B adds classic SAP APIs that are documented and generally upgrade-stable. Level C accesses SAP internal objects and carries upgrade risk. Level D uses techniques SAP explicitly does not recommend, such as modifications, implicit enhancements or write access to SAP tables.
- BeforeClassic extensibilityAnything goes: user exits, modifications, direct table access; upgrades require SPAU and regression across the board
- ABAP Cloud eraThree tiersTier 1 ABAP Cloud, Tier 2 wrappers, Tier 3 classic ABAP; a build model more than a risk scale
- August 2025Clean core levels A to DGraded by what an extension uses; classic APIs get their own level; ATC checks support classification
| Question | Three tiers | Levels A to D |
|---|---|---|
| What is rated? | How the code is built | What the extension uses and how upgrade-stable that is |
| Classic BAPI call | Tier 3 (classic ABAP) | Level B if the BAPI is classified as a classic API |
| Direct write to an SAP table | Tier 3 (classic ABAP) | Level D |
| Released API from ABAP Cloud | Tier 1 | Level A |
| Use for governance | Coarse: most legacy code is Tier 3 | Graded: shows where the real upgrade risk sits |
What would you do?
Grace asks Kenji for a single sentence for the steering committee on why the program should report clean core levels instead of tiers. Which sentence is best?
- Clean core keeps S/4HANA upgrade-stable while allowing differentiating extensions, side-by-side or on-stack.
- The three tiers described how code is built: ABAP Cloud, wrappers, classic ABAP.
- In 2025 SAP evolved them into levels A to D, rated by what an extension uses.
- Levels split the old Tier 3 into classic API use (B), internal objects (C) and non-recommended techniques (D).
Check your understanding · not graded
Lesson check
1. What does a clean core level rate?
2. In the three-tier model, what was Tier 2?
3. Why did the old Tier 3 tell Contoso's board little about risk?
4. Which level covers extensions that use only publicly released, stable SAP interfaces?