RDS for Oracle database architecture
The multitenant architecture enables an Oracle database to function as a
multitenant container database (CDB). A CDB can include customer-created
pluggable databases (PDBs). A non-CDB is an Oracle database
that uses the traditional architecture, which can't contain PDBs. For more information about the multitenant
architecture, see Oracle Multitenant Administrator’s Guide
For Oracle Database 19c and higher, RDS for Oracle supports the single-tenant configuration of the multitenant architecture. In this case, your CDB contains only one PDB. The single-tenant configuration of the multitenant architecture uses the same RDS APIs as the non-CDB architecture. Thus, your experience with a PDB is mostly identical to your experience with a non-CDB.
Note
You can't access the CDB itself.
In Oracle Database 21c and higher, all databases are CDBs. In contrast, you can create an Oracle Database 19c DB instance as either a CDB or non-CDB. You can't upgrade a non-CDB to a CDB, but you convert an Oracle Database 19c non-CDB to a CDB, and then upgrade it. You can't convert a CDB to a non-CDB.
For more information, see the following resources: