Use this technique to upgrade multitenant architecture Oracle Database releases (Oracle Database 12c Release 1 (12.1.0.1) and later by upgrading container databases (CDBs), and then upgrading multiple pluggable databases (PDBs) in parallel.

• About Upgrading Pluggable Databases (PDBs) In Parallel
  Using the In-Parallel technique, you can upgrade the CDB, and then immediately upgrade PDBs using parallel SQL processors.
• Upgrading Multitenant Container Databases In Parallel
  Use this technique to upgrade CDB$ROOT, PDB$SEED, and all PDBS in the CDB in one upgrade operation.