When you switch on the add-on for Xero Tracking Categories, you will see an extra tab when you create/open a job -- so you'll have the Job Details, Quotes & Invoicing, and the extra tab 'Custom Fields' for Xero Tracking Categories as shown below.
Xero has tracking categories that help identify invoices for reporting. This add-on allows you to set jobs with a tracking category in ServiceM8 that will be set to in Xero.
Click ServiceM8 Add-ons
From the ServiceM8 Add-ons page, scroll down until you find the Xero Tracking Categories add-on.