AI generated answer (using internal Slack threads):

In NetSuite, the tax codes that are available for selection on a transaction are determined by the nexus of the subsidiary associated with the transaction. If your Canadian subsidiary has a US nexus, it should be able to use US tax codes. However, if you're unable to select a US tax code for a sales order with a US customer, it could be due to the following reasons: 1. The customer record might not be associated with the correct subsidiary. Ensure that the customer record is linked to the Canadian subsidiary with the US nexus. 2. The nexus setup for the Canadian subsidiary might not be correctly configured. Check the nexus setup under Setup > Company > Nexus to ensure that the US nexus is correctly associated with the Canadian subsidiary. 3. The tax code setup might not be correctly configured. Check the tax code setup under Setup > Accounting > Tax Codes to ensure that the US tax codes are correctly set up and associated with the US nexus. To configure the system to allow the selection of a US tax code for this transaction, you would need to ensure that the customer record is associated with the Canadian subsidiary that has a US nexus, and that the US tax codes are correctly set up and associated with the US nexus. Remember, when changing the address on an existing transaction, NetSuite may not allow the change if it requires a different tax form. In such cases, you may need to create a new transaction with the correct address. ****** Disclaimer: This response was generated by an artificial intelligence assistant based on previous conversations from multiple Slack channels, and may contain inaccuracies or mistakes. ******