Let me share details about customised form style contents in QuickBooks Online (QBO), @Sine Wave Power.
In QBO, Business Reg No. and Business ID No. terms are of the same meaning. For the time being, the Business Reg No. title will automatically change to Business ID No. when previewing or printing your sales documents.
On the other hand, you're utilising the VAT feature in QBO to display your customer's VAT numbers. This is why you're required to update your tax details to activate it. Since your business is not registered, simply select the I don't need a VAT Registration No. option to get started and continue creating your sales document. Please see the screenshot below for your reference.
Additionally, we've recently updated the VAT Feature in accordance with the South African new rates to ensure a smooth tax year-end for our customers. To answer the most frequently asked questions about these changes, you can refer to this article: VAT Feature Update FAQ.
Also, I'm adding this article to further guide you in managing your business growth using QBO: QuickBooks Help Articles. It includes topics about account management, banking, and expenses and vendors, to name a few. You can click the + More topics button to view other selections.
Please don't hesitate to let me know all of your QuickBooks needs, I'm always happy to help. Have a great day, and keep safe.