Custom Code Migration
What it is
The structured process of getting existing custom ABAP cloud-ready: run the ABAP cloud-readiness ATC variant (often via the Custom Code Migration Fiori app / ATC on a central checks system), triage findings, and remediate to released APIs and allowed syntax.
Why it matters for Clean Core
It's the concrete shape of 'becoming Clean Core' for a brownfield estate. Done well it's prioritised (worst findings first) and incremental, not a big-bang rewrite.
Common pitfall
Trying to fix everything at once, or fixing syntax without fixing the unreleased dependency underneath. Prioritise by ATC severity and by which objects you actually still run — dead code gets retired, not migrated.