ABAP Language Version
What it is
A per-object setting (Standard / ABAP for Key Users / ABAP for Cloud Development) that fixes which language constructs and APIs an object may use. 'ABAP for Cloud Development' is the strictest and is what ABAP Cloud requires.
Why it matters for Clean Core
It's the dial that makes Clean Core a compile-time guarantee rather than a guideline. Set it to Cloud Development and the syntax checker rejects unreleased APIs and obsolete statements before you can ship them.
Common pitfall
Leaving new objects on 'Standard' to 'avoid the errors'. That just defers the migration and lets classic patterns leak back in. Author new code at the Cloud Development version from day one.