When setting up a bank transaction rule, why is the Tax field mandatory? What impact does this have on the rule or the transactions when it is matched?

Thanks for posting here in the Community, @crossbat.

When using the international version of QuickBooks Online you'll see the Tax field for bank transactions. This is used to track those transactions with taxes. 

If that's the case, you can either choose the Exempt (0%) or No Vat (0%) option by clicking the Tax drop-down menu. These are the options that you can select if you're not using any tax for a certain transaction.

In addition to that, let me share this article to guide you use bank rules in QuickBooks: Set up bank rules to categorize online banking transactions.

Please let us know if you still need help with this matter. I'll be around together with my colleague to continue to help you out. Take care always and have a good one.

Thank you. Am I right in thinking that the tax and category is only relevant when Adding a new transaction? If I’m matching to an existing transaction in QBO, then it doesn’t matter what I put into these fields (or I can ignore them completely) because QBO will take the category and VAT from the original transaction?

Thanks for getting back to us crossbat - that's right, you don't need to complete the fields when matching as these will be overridden in the system with the details of the matched manual transaction. :)