system catalogue contains system tables and other built-in objects such as data types, functions, and operators.
Modifying the system catalogue
In principle, the system catalogue is not intended to be user-modifiable.
Beginning with PostgreSQL 12, it is now possible to cleanly run ALTER TABLE on system catalogue tables (commit 590a8702); use-case for this situation is to modify storage parameters (reloptions), e.g. fillfactor or autovacuum settings.