Xero allows you to create “allowance” Pay Items, but these can only be applied during Pay Runs, not on Timesheets. As a result, they can not be imported from other systems, such as Tanda.
A workaround is available — simply create your allowance as a regular, “Ordinary Time” Pay Item, with a name that matches the export name in Tanda. Ensure that the unit type is set to “Hours”, so that it works with Xero timesheets. The data can then be correctly imported from Tanda to your Xero timesheets.
While this will generally not cause any other issues, though it may look a bit odd on payslips. You may wish to consult your accountant or bookkeeper if you use this workaround.