Question regarding Check Digit Routines

Good afternoon,

I'm trying to understand the mathematical method used when creating check digit routines, used in computer software applications to confirm that a reference entered by a customer is valid.

I know why we have them, but don't understand the workings behind them for example Weightings and the Modulus.

I need to work out how to create new ones but need to understand the above in order to complete them.

Do I have the correct forum, if not could you recommend another site, thanks.