No Key Mapping Behaviour

When no key mapping is defined the logic followed to determine whether a record in inserted or updated is based on this decision tree:

For ServiceNow->ServiceNow migrations it is recommended that No Key Mappings be used whenever the migration method is set to Import Set (Scripted) with the Migrate Sys-IDs option because matching on Sys-Ids is generally more reliable and efficient than matching on any other combination of fields.