Weighting from the left
An EAN-13 carries 12 digits of payload and 1 check digit. Weights alternate 1 and 3 starting at the leftmost digit, which means the digit next to the check digit is always weighted 3.
The same arithmetic covers UPC-A, EAN-8, GTIN-14, and ISBN-13. Only the payload length changes, which is why a UPC-A number padded with a leading zero keeps a valid EAN-13 check digit.
Assigning a number
Add check digit mode takes the 12-digit body and returns the full number. This is the step that turns a GS1 company prefix plus an item reference into a scannable barcode number.
A valid check digit does not mean the number is registered to you. Retail numbers have to be allocated from a prefix your organization owns, or two products end up sharing a barcode.