From DBSight Full-Text Search Engine/Platform Wiki

Primary Key Column
The field that contains a value that can differentiate the document from others.
Effect: This will prevent duplicated documents.
Modified Date Column
The field that contains a date value of when the document is modified.
It can also be an incremental integer that later created or modified document has a larger value.
Effect: This will record when the document is updated. So DBSight knows when to refresh it, if the document also has a Primary Key Column.