When patching, you have two entities in your environment
  - Oracle Database Software
  - Oracle Database

You can check the patches installed using opatch utility which you can find in $ORACLE_HOME/OPatch:

./opatch lsinv -bugs_fixed | grep -i 'database cpu'

Following patching your database with a PSU/CPU, you want to keep a history of the patches applied.
In 11g (actually this started since 10.2.0.4), there is a new post patch installation script which does the job.
It is catbundle.sql, and is located in $ORACLE_HOME/rdbms/admin. (In previous releases this was done by catcpu.sql)
This shall be invoked with 2 parameters, patch type (PSU/CPU) and action (apply or rollback).
The purpose of the script is to update the entry in registry$history.

SQL> @catbundle.sql CPU APPLY
PL/SQL procedure successfully completed.

PL/SQL procedure successfully completed.

1. PSU (and CPU) patch README already contains information about when to execute post install step (i.e loading modified .sql files, ex: catbundle.sql) on databases created using DBCA.

2. PSU/CPU entry in "registry$history" gets updated by the patch post installation step (i.e catbundle.sql). Skipping this step (where it is not actually MANDATORY in above mentioned case) results no PSU/CPU entry in registry$history. Even though it is not mandatory to run PSU/CPU post install step (i.e catbundle.sql), there is no harm in running as it helps to update the registry$history.


References:
How to confirm that a Critical Patch Update (CPU) has been installed in Linux / UNIX [ID 821263.1]
Is it required to run PSU/CPU patch post installation step after creating the database using DBCA in already patched Oracle Home ? [ID 1388555.1]
Best approach for Oracle database patching sequence to latest/required patchset along with CPU/PSU/any-other-one-off patch [ID 865255.1]