Serial numbers can take various formats. One of the most popular is a set of four or more fixed-length alphanumeric blocks separated by dashes. This structure allows the code to be long and at the same time easy to read and rewrite.
When you want to give someone a gift card or voucher, it is a good idea to have it marked with a unique code that will allow you to easily verify whether the voucher is valid or has not already been used. Such codes should be relatively short so that they can be dictated quickly, yet random enough so that someone cannot guess the value of another voucher that is not theirs.
If you intend to distribute the codes worldwide, and at the same time want to be able to easily identify which country a code originates from, you can add the country code to each code as a prefix. For example, for codes for the USA, add the prefix US-, for England UK-, and for Germany DE-, etc.