CPU stands for Critical patch Updates introduced in January 2005 to provide security fixes to help customers lower their security management costs. Generally if you have only one database installed then checking the database inventory will give you the latest patch details. But,  if we have multiple database in single oracle home then it might not give correct results. There might be a chance that one DB is applied with latest patches and others are not. In such cases, we need to check last CPU applied by logging into the database.

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col VERSION for a15;
col COMMENTS for a50; 
col ACTION for a10; 
set lines 500;
select ACTION,VERSION,COMMENTS,BUNDLE_SERIES from registry$history;