Can someone shed some light on the differences between exclude.fs and
exclude.dir when it comes to *NIX systems ?

I have a client wondering why the statement:

        exclude.fs /mnt/raidC

did not work/still backed up this filesystem but when I suggested he use
exclude.dir, it worked ?

I understand the issue of what happens when you add an exclude.fs *after*
you already backed up the fs accidently (I did read through the book).