Tier 1 / 2 / 3 Extensibility
What it is
SAP's three-tier model for how cleanly an extension is coupled to the core. Tier 1 = on-stack developer extensibility using only released APIs (ABAP Cloud). Tier 2 = on-stack 'cloud API enablement' / classic extensibility kept within guardrails. Tier 3 = side-by-side on SAP BTP, fully decoupled.
Why it matters for Clean Core
It gives you a vocabulary to defend an architecture choice. Tier 1 is the cleanest; Tier 3 is the most decoupled. Knowing which tier a requirement belongs in is half of a Clean Core design review.
Common pitfall
Confusing the developer 'Tier 1/2/3' model with the older 'key-user / developer / side-by-side' wording. They overlap but aren't identical — Tier 2 is specifically the on-stack 'still classic but governed' middle ground.