Customers and contacts are imported from Xero into QuickBooks Time. Customers (contacts that have been invoiced) are automatically added via import, but contacts must be added this way: In QuickBooks Time, in the top right, select Xero > Preferences > Manage Xero Customers > find and select your Xero contacts you want imported in as QuickBooks Time customers, then select Add Customers.
You can assign all customers to all team members during setup, or not, by choosing Assign imported customers to all employees. This option can be selected/deselected later for future imported customers. Individual assignments can be changed by going to Customers > select the pencil icon > check or uncheck Assign to all team members (if unchecking, select Edit to assign individuals).
If inventory items are imported into QuickBooks Time, they will become a selection on timesheets. This field can be made required by going to the Xero dropdown > Preferences > check Require employees to select an Inventory Item. An inventory item can only be imported if it is not marked with "I track this item" in Xero.