There are three PDBs shown. CDB1, which is an Oracle Database 12.2 release, CDB2, which is an Oracle Database 18c, release, and CDB3 which is the new target Oracle Database release. CDB1 has two PDBs, pdba and pdbb. CDB2 has two PDBs, pdbc and pdbd. You can either plug in both PDBs from one source (for example, both PDBs on CDB1, which are release 12.2), or you can plug in one PDB from CDB1 (12.2) and one PDB from CDB2 (18) into the target release, CDB3.