PAN structure

While browsing through old mails, I found couple of mails that talk about different coding schemes. Here is the first, detailing the structure of PAN.
(PS: PAN stands for Permanent Account Number, hence it's meaningless to say "PAN Number". I've heard people asking for "PAN Number" :-s)

[mail contents]
Permanent Account Number (PAN) is a unique ten digit all India number allotted to an assessee, for identification. The computed system generates a 10 character PAN using he information furnished by the assessee in prescribed form No.49A. PAN has the following structure:

PAN: AAA S N 0000 C

Where,
AAA = Alphabetic series running from AAA to ZZZ
S = Status (i.e. Company)
N = First character of assessee's Last Name/Surname
0000 = Sequential Number running from 0001 to 9999
C = Alphabetic Check digit

The unique PAN is allotted under each alphabet and in each status, i.e. individual, HUF, Firm, Company, Trusts, Body of Individuals, Association of Persons, etc.

For example, the PAN of an individual assessee viz. Jhaveri Gautam Nanubhai is ADAPJ5993J. In this number the character P identifies the status of the assessee (i.e. status of Individual) and J is the first letter of assessee's surname.

Similarly if the status of an assessee HUF, for example say Vinod Modha(HUF), his PAN would be AACHV4018B. In this number the character H identifies the status of the assessee i.e. HUF and V is the first letter of assessee's name i.e. Vinod.

In the same manner if the status of an assessee is Company, for example say Lok Prakashan Ltd. its PAN would be AAACL2742F. In this number the character C distinguishes the status of the assessee i.e. Company and Lis the first letter of assessee's name i.e. Lok Prakashan Ltd.

The different characters used for different status is as under:

Individual : P
Company : C
HUF : H
Trusts / Association of Persons : A
Firm : F
Body of Individuals : B

[/mail contents]
(I had received this mail as an scanned image of paper. But I've put the information in plain text, so that it is accessible through search engines.)

As per the details given on http://www.simpletaxindia.org/2007/11/know-your-pan-structure.html

status field for -
Individual : P
Company : C
Trust : T
Partnership Firm : F
Body of Individuals : B
Association of Persons : A
HUF : H
Local Authority : L
Govt : G
Judicial Artificial Person : J


The above mentioned link has more examples for clarification.

I don't have any details about : how the check character (10th) is calculated from the first nine alphanumeric characters and what's the reason for having 2 sequential series (1st AAA-ZZZ & 2nd 0001-9999).
If any one of you have any idea, please let me know.