This is a mathematically computed "check digit" sort've like a checksum.
It can be used to compare the validity of the other characters in the VIN.


  Here's how you compute it:  (From http://www.intnet.net/pub/AUTO/VIN.info)

Assign to each numeric in the VIN its actual mathematical value and assign 
to each alphabetic the value specified below:                

A=1, B=2, C=3, D=4, E=5, F=6, G=7,    H=8, J=1, K=2, L=3, M=4, N=5, P=7, 
R=9,   S=2, T=3, U=4, V=5, W=6, X=7, Y=8, Z=9.

Multiply the assigned value for each character in the VIN by the weight 
factor specified for it below:                

1st=8, 2nd=7, 3rd=6, 4th=5, 5th=4,6th=3, 7th=2, 8th=10, 9th=0 (check digit),
10th=9, 11th=8, 12th=7, 13th=6, 14th=5,15th=4, 16th=3, 17th=2.

Add the resulting products and divide the total by 11. The remainder is 
the check digit. If the remainder is 10, the check digit is X.
                        
Example VIN Characters: 1  G  4  A  H  5  9  H  4  5  G  1  1  8  3  4  1 

       Assigned Values: 1  7  4  1  8  5  9  8  4  5  7  1  1  8  3  4  1 

           Multiply by: 8  7  6  5  4  3  2  10 0  9  8  7  6  5  4  3  2

Add products: 8+49+24+5+32+15+18+80+0+45+56+7+6+7+6+40+12+12+2=411 Divide 

by 11: 411/11 = 37 4/11 Check digit:4 (compare to character in 9th position)

The check digit (9th position) will always be a numeric or an X.