When you delete records from Salesforce object, they are moved to Recycle Bin and these records are said to be soft deleted. The soft deleted records can be restored.
When you delete records permanently from Salesforce object, they are said to be hard deleted. The hard deleted records cannot be restored.
To enable hard delete of salesforce records from IICS, check the Hard Delete option when you select the Delete operation in Target transformation. If the option is not selected, the delete operation is considered as soft delete.