The Billing Zip Code field in our Transaction log report shows the exact billing zip code used to calculate taxes. It is being filled by the user when the Unites States or Canada are picked as the billing country. For other countries, it is being filled automatically based on the user's IP address.
We are not able to determine the exact billing zip code by IP the IP address, and that is why this billing code may be filled only partially with its first part (e.g. SE6 only for a 6-character zip code).
We also ask users to provide their Postal (ZIP) code in the credit card details form during checkout, but this data is not being taken into account when the Transaction log report is being generated. This information relates mostly to the user's payment token information and not to the user's transaction.