What is the purpose of the coalesce field when importing data?
The coalesce option allows you to update existing target table records when transforming import data.
The coalesce option on a field map allows you to specify if the selected Target field should be used to coalesce on when import set records are transformed. If the field map Coalesce checkbox is selected, when the import set row is transformed the instance checks for an existing record in the target table that has the same value in the Target field as the import set row Source field.
https://www.servicenow.com/docs/bundle/xanadu-integrate-applications/page/administer/import-sets/concept/c_ImportSetCoalesce.html