available in the master server properties:
then as far as I'm aware there is no difference - altho' they will be saved to different tapes if you use more than one (NB doesn't mix retentions on tape by default). I presume that they are there and all set to infinity by default, allowing you to change them to whatever period you want. Just pick one for use & stick with it. (We chose the first available (9))
***EDIT***
The following is from the Admin Guide & clarifies the above somewhat:
"...
The Retention periods list displays a list of the current definitions for the 25 possible levels of retention (0 through 24). By default, levels 9 through 24 are set to infinite. Retention level 9 is the only level that cannot be changed and remains at infinite.
Note that if left at the default, there is no difference between a retention level of 12 and a retention level of 20, for example.
..."