SID vs Service_Name

Question

Came across these entries (example) in TNSNAMES.ORA on a client machine; and need some clarifications from the gurus here.
10g TNSNAMES.ORA Server 123.456.789.111
DEV.SERVER.WORLD.COM =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.111)(PORT = 1521))
    (CONNECT_DATA =
      (SID = DEVWORLD)
      (SERVER = DEDICATED)
PROD.SERVER.WORLD.COM =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.111)(PORT = 1521))
    (CONNECT_DATA =
      (SID = PRODWORLD)
      (SERVER = DEDICATED)
10g LISTENER.ORA (Server 123.456.789.111
SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = DEVWORLD)
      (ORACLE_HOME=C:\oracle\product\10.2.0\db_1)
    (SID_DESC =
      (SID_NAME = PRODWORLD)
      (ORACLE_HOME=C:\oracle\product\10.2.0\db_1)
     (SID_DESC =
      (SID_NAME = BLAHBLAH)
      (ORACLE_HOME=C:\oracle\product\10.2.0\db_1)
11g TNSNAMES.ORA (server 123.456.789.000)
PRODSRV =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.000)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = PROD.SERVER.WORLD.COM)
11g LISTENER.ORA (Server 123.456.789.000)
SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = C:\Oracle\product\11.2.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:C:\Oracle\product\11.2.0\dbhome_1\bin\oraclr11.dll")
    (SID_DESC =
      (SID_NAME = PROD)
      (ORACLE_HOME=C:\Oracle\product\11.2.0\dbhome_1\)
Questions
1) Server level
In my 10g example, it looks like the link from LISTENER and TNSNAMES is SID_NAME. However, on the 11g, I'm seriously not sure how does it link considering SID_NAME is not on the TNSNAMES.
a) How does one suppose to specify TNSNAMES to correspond with the LISTENER so connections can be established?
b) Also, can 2 same SID exists on the same server?
c) What is that CLRExtProc on the 11g Listener? I don't think I have that database.
Client TNSNAMES.ORA
PRODSRV =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.000)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = PROD.SERVER.WORLD.COM)
DEVWORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.111)(PORT = 1521))
    (CONNECT_DATA =
      (SID = DEVWORLD)
      (SERVER = DEDICATED)
PRODWORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 123.456.789.111)(PORT = 1521))
    (CONNECT_DATA =
      (SID = PRODWORLD)
      (SERVER = DEDICATED)
2) Server - Client level 
a) how does one normally sets up client connections?
As you can see from my client machine, some of the entries are with SID and some don't.
I can connect using alias the following: SQLPLUS SYS@PRODSRV or @DEVWORLD  or @PRODWORLD
b) I cannot connect even if I issue SET ORACLE_SID=PRODWORLD and then try SQLPLUS SYS as SYSDBA. Is this method used when connecting within server level?
Thanks.

Answer 1

Questions
1) Server level
In my 10g example, it looks like the link from LISTENER and TNSNAMES is SID_NAME. However, on the 11g, I'm seriously not sure how does it link considering SID_NAME is not on the TNSNAMES.
This isn't how this works. TNSNAMES.ora is a set of entries fore resolving service calls to connect to the database. You can connect through a SID or a SERVICE_NAME. Listener.ORA is to specify Listener parameters and how/where it will listen to connections.
a) How does one suppose to specify TNSNAMES to correspond with the LISTENER so connections can be established?
Do a lsnrctl status <listener_name> to get all the information of how the listener is working. What port and what SID/SERIVCE_NAME. From there set your TNSNAMES accordingly.
b) Also, can 2 same SID exists on the same server?
No. And I don't know why you would attempt this.
c) What is that CLRExtProc on the 11g Listener? I don't think I have that database.
This is for .NET extenstions and is configured automatically, don't need to concern yourself with it.