CPU = Critical Patch updated/upgrade
PSU = Patch set update/upgrade
Before 11g oracle release the CPU patches Quarterly and after that it came as PSU bundle. you can apply the latest PSU for your DB version 11g Onward.
Thank you but this info does not answer any of my questions.
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Correct - once you apply a PSU, you can only apply future PSUs (and not CPUs, now renamed to SPUs) - PSUs include CPU/SPU content, so it is recommended to apply them rather than CPU/SPU.
See section 9.1 in MOS Doc 854428.1
Thanks for the verification Srini!!