These notes all come from "About the Multitenant Architecture".
A container is either a PDB or the root. The root container is a collection of schemas, schema objects, and nonschema objects to which all PDBs belong.
Every CDB has the following containers:
- Exactly one root
- The root stores Oracle-supplied metadata and common users. An example of metadata is the source code for Oracle-supplied PL/SQL packages. A common user is a database user known in every container. The root container is named CDB$ROOT.
- Exactly one seed PDB
- The seed PDB is a system-supplied template that the CDB can use to create new PDBs. The seed PDB is named PDB$SEED. You cannot add or modify objects in PDB$SEED.
- Zero or more user-created PDBs
- A PDB is a user-created entity that contains the data and code required for a specific set of features. For example, a PDB can support a specific application, such as a human resources or sales application. No PDBs exist at creation of the CDB. You add PDBs based on your business requirements.