Similar monitors in different targets

Hi, have a doubt

For example, when I scope Monitors to below targets,

1. Logical disk (Server)

2. Windows server 2003 Logical Disk

3. Windows server 2008 Logical Disk

I couldn't see any unit monitors in 1st one i.e. Logical disk (server), but in 2nd & 3rd .  Is that cause of first one an abstract.  And what if I need to point to all 2k3 & 2k8 computers, should I target both versions of windows separately or shall I point to 1st one, i.e. Logical Disk (Server).  Similar situations are there like physical disk etc.,  Can anyone let me know difference between 1st & 2nd, 3rd mentioned above.

Thanks!!