What are the patches we cannot merge it?

Don't merge RDBMS patch with apps patch OR AD patches and non-AD patches cannot be merged.
Don't merge patches of different version (11.5.x, 12.1).
Don't merge ATG family product patches.

Note* : But we can merge language patches,pre-requisite and co -requisite patch into one patch.

What is ERP?
Click on this to see in details  http://abduulwasiq.blogspot.in/p/ebs-11i-r12.html