The manual for aufs makes several references to something called a "whiteout", but it's not clear to me what this is.
What is a whiteout and why would one exist?
It is how the union file system supports deleting from a union where the underlying file to be deleted resides on a read-only file system.
Say you have
all union mounted together in
And you do
It would place a whiteout for
See State of the unionfs for a good read and a full explanation.