NetVault Question

I've been out of the network admin side of things for two years. I've
recently started working as a net admin for a company and one of my
primary responsibilities is backup. When I was responsible for this I
was using Legato and Arcserve, backing up to tape. In the new
position I will be setting up Netvault Version 7.10 Build 42, and
using a jetstore 2TB raid box. Up until two weeks ago I hadn't even
heard of a VDL. My question is this, when setting up the devices in
the device mgmt window, how do I know whether to do a library or a
standalone device. And for that matter how, how do I figure out how
many drives and slots?

Thanks for the help,
Phil Puckett