Go to the Settings tab in the top right corner of Xero.
Choose General Settings from the drop-down menu.
When the page opens, look under Reports section on the right side and click on Tracking.
In Tracking, you can easily add category options for your marketplace tracking category to match your Amazon Marketplaces.
Once you have added the tracking, go back to A2X and refresh Xero cache in Settings > Connections.
After cache is refreshed, the tracking options will become available in Settings > Invoices. You will see them in the drop-down list.
Finally, click on the orange 'Update Invoice Settings' button.
Now, the settlements will be tracked by the marketplace they are from when you send them over to Xero,