Question on symbol versioning

["Amker.Cheng" <amker dot cheng at gmail dot com>]

Hi:
   I am studying elf symbol versioning referring document from following address
http://lists.debian.org/lsb-spec/1999/12/msg00017.html
   However, at "Symbol lookup" section, for the second case, there are words
like:
----------------------------------------------------
In case only the object file with the reference does not use
versioning but the object with the definition does, then the reference
only matches the base definition.  The base definition is the one with
index numbers 1 and 2 (1 is the unspecified name, 2 is the name given
later to the baseline of symbols once the library started using symbol
versioning).
----------------------------------------------------
I am puzzled about

1 : "The base definition is the one with index numbers 1 and 2"
Question :
What does the index number mean, vd_ndx in Elfxx_Verdef?
How could a base definition has two index numbers?or both definitions
with index number 1 or 2 are base definition?

2 : "1 is the unspecified name"
Question :
What does the unspecified name mean?


I am really lost in it, please help me out, Thanks very much.

-- 
Best Regards.