If you performed data migration previously and want to transfer only new entities or those that changed since the initial migration, use the "Skip previously migrated data" option at the Transfer step (the 6th step) of the migration wizard.
This option is convenient if you first ran the initial data migration, tweaked X-Cart 5 to get it ready for production, and now decide to go live. In this case, most data is already on X-Cart 5, and you only need to sync the latest ones.
If you migrated some entity (product, order, user, etc.) and then changed it on the X-Cart 5 side, while the X-Cart 4’s version is still the same, the wizard will skip this entity with the ‘Skip previously migrated data’ option enabled.