Knowledge Base Help & Documentation:
Why is it recommended to clear current data in order to restart migration?
Frequently, the migration should be restarted because something is wrong with the data on a Target CMS. It can often be damaged by the 3rd party extensions, inappropriate CMS installation, etc. That’s why launching a new migration filled with corrupted or modified data is not recommended to avoid the risk of data mixing/duplicating. Therefore, it is strongly advised to clear current data from the Target CMS before migration. When all the corrupted/modified data is deleted – the Source data are moved to the clear Target CMS, preserving relationships between entities.
Please note! Clearing data from a Target CMS is irreversible option, so make sure to backup your website data before restarting the migration.