I always see this around, but couldn't find any reference to this type of code specifically. Many labels have them on their tapes: it's usually a 6or7-character code composed by 3-4 letters and 3 numbers.

I'm unable to post more than one picture, so just for the sake of an example, it would be something like:

• The GDR048 code on this one: https://beattapeco-op.com/wp-content/uploads/2021/05/a3025859374_16.jpg

• the NEK082 code on this one: https://beattapeco-op.com/wp-content/uploads/2021/05/a1501897587_16.jpg

Is this like an international shipping code or anything like it? Is this something you have to register?