Networking  DNS dig command usage

(f)	Im a bit confused as to the interpritation of the output various
levels of the domain space when doing dig on:  

Execute: # dig @A.ROOT-SERVERS.NET spectral.mscs.mu.edu A         
Execute: # dig @spool.mu.edu spectral.mscs.mu.edu A

What is the main
difference?

On the first line, you are querying a root nameserver,
'A.ROOT-SERVERS.NET'.  Root nameservers are only configured to direct
clients to the nameservers for the top level domain of your request. 
In this case, 'edu'.

High level nameservers usually only give responses for the domains
they manage, they will not always give answers for random hosts on the
Internet.

On the second command, you are asking the nameserver 'spool.mu.edu',
and I'm guessing that is a recursive, resolving nameserver.

Take a look at the output of:
dig spectral.mscs.mu.edu A +trace

Notice the answers at each step, and specifically the line that
specifies where the answer came from:
;; Received 139 bytes from 134.48.25.32#53(DNS2.mu.edu) in 113 ms