As soon as you connect ServiceM8 to Xero, all customer contacts in Xero are pulled into ServiceM8 (suppliers are ignored). Xero continues to push any new or edited contacts to ServiceM8 approximately every 30 minutes.
A client created in ServiceM8 is not sent to Xero immediately. The first time you approve an invoice for that client, the client record is created in Xero with the invoice. From that point on, updates made in either program flow to the other approximately every 30 minutes.
After the initial link, edits to names, phone numbers, addresses, and other billing contact fields in either system update the matching record in the other on each sync cycle (about every 30 minutes).