With bash as my shell, a simple way to demonstrate my question is this:
mkdir -p foo.app/Contents cd foo.app/Contents echo -n 'AEgxFXTC' > PkgInfo ln PkgInfo f
The ln will fail with
ln: f: Operation not permitted
I am on 10.10.5 with a Mac OS Extended (Journaled, Encrypted) file system.
There seems to be some magic squirreled away in the file system for files named PkgInfo inside a directory named Contents. The contents of the file does not seem to matter.
What is the magic specifically? Are there other magic names besides PkgInfo? Why is it special? etc.