With Practice Ignition integrated to Karbon, new contacts (and limited details) are created when a match doesn't occur. The contact data added (for an organization or person) includes only contact name. The work assignee on the work item is assigned based on what is selected in management console under Settings > Connected Apps > Practice Ignition.
If you would like to have more data flow from Practice Ignition to Karbon, we recommend using Zapier to move that preferred data once the proposal signed. An example of a zap that moves that data for organizations can be found here.