In the EPM world, there are two types of patches: Patch Set Updates (PSU), and Patch Set Exceptions (PSE).
A Patch Set Exception is a patch that Oracle creates as a one-off to address a specific bug, and is provided to customers that are in critical need. PSEs are neither version controlled nor cumulative. They are not tested in every customer situation. Usually these patches are only installed with Oracle’s guidance after opening a service request and Oracle has identified a bug. These usually have long, nasty numbers associated with them, such as 126.96.36.199.1788341
Patch Set Updates are proactive, packaged patches that have been certified and tested by Oracle and are recommended for installation in all cases. Each PSU can contain 25-100 individual bug fixes, performance enhancements, and even minor new functionality. These numbers are usually nice and round, like 188.8.131.52.500 or 184.108.40.206.501.
In general, you should only apply PSEs if instructed to do so by Oracle in response to an identified bug. You should never apply PSEs simply because they are available. Because they are designed for a specific use case, PSEs have been known to break otherwise perfectly functioning systems.
PSUs, on the other hand, should always be applied.