A financial institution code (bank code) is a 4-digit number assigned to a financial institution, along with a 3-digit number assigned to each of its branches.
This information is required on pixiv to receive earnings from Requests and other payments.
You can look up your bank code at the link below.
Find out your branch name, account type, and account number.
*If you have any questions about the information required for bank transfers, please contact your financial institution directly.