The Xero integration does not automatically export metadata (such as attachments, notes, Xero Categories, etc.) to Xero.
You must export metadata to Xero by go into Report in your left-hand-side menu, click on Xero, select which transactions you want to export metadata for, and then press the Export to Xero button. They will then show as Pending until Xero accepts or declines the data. Transactions with metadata which have not been exported to Xero will remain in the To Export tab, and those which have successfully been exported to Xero will move to the Exported tab.
Transactions are required to have a Xero Category selected, and you can enable/disable making any List (such as custom Tracking Categories imported from Soldo, Suppliers, Customers, etc.) a required field.