When you submit a receipt, invoice or other piece of financial paperwork from a new supplier, Receipt Bank will take a best guess at how that document should be categorised and assign an appropriate category.
As more items from that supplier are submitted and reviewed, Receipt Bank will learn how you like to categorize that supplier and start defaulting to that category automatically.
If you're using a Receipt Bank Business Plus, Premium, Streamline or Optimize account, you can view suppliers who have had default categories applied to them from the 'Suppliers' tab.
You can turn off this autocategorization by navigating to the general tab of the 'Account Settings' page and turning the 'Autocategorization' toggle to 'OFF'. Please note that if you need to have the codes applied for known suppliers you need to keep the "Obey supplier rules" toggle on (it appears when you turn off the "Autocategorisation").