A question about volumeRetention

Hi everyone
I am new t Bacula, and I have problem understanding some concepts, one
of them is the idea of volumes, how they are created and
VolumeRetention
I do understand that it is from the backup-to-tape world, and it is
basical a representation of a tape, right??
First of all, i probably should mention that i am backing up to disk,
not to tape, anyways  i thought that Volume retention tells Bacula how
long to use a Volume and when to create a new one, i.e a
VolumeRetention of 6 days means Bacula will create a new volume file
to use very 6 days

But this does not seem to be the case, from what i have seen in my
tests. So if I want a new volume to be created every 7 days for weekly
backups(one full, 6 increments) the only directive to use is "Volume
Use Duration" like  "Volume Use Duration=6 days 20 hours"??
What happens to the previous volume, will restore still work as it should??

Regards /Mehrdad