Understand how the FHIR server updates _lastUpdated whenever a recxord is updated.

In the Data Ingestion Guides there is a lastUpdated field. How is this used?

The lastUpdated field is part of the meta data element in a FHIR resource. SAFHIR updates this field when a FHIR resource is created or updated. This is used in many searches to allow a search query to only return the records that have been updated since a specified time that is included in the search parameters.