The method for importing data from Excel or CSV files into existing apps has been updated.
This article explains the main differences between the classic method and the new method.

With the classic file import method:

If a single instance of data that cannot be imported is found in a file, the import process will stop and none of the file's data will be imported.

With the new file import method:

You can select how to handle errors on the Import from file screen.
For details, refer to the following page. Options for handling errors during file import

With the classic file import method:
The Imported files screen shows the details of the first error that was encountered during the file import process.

With the new file import method:
The Imported files screen shows the number of rows that were not imported.
You can also download a CSV file with the rows that were not imported, along with the reasons they were not imported.
After checking the error details and correcting the data in the CSV file, you can use the file to import the data sets that were not imported during the import session.

With the classic file import method:
A Lookup field cannot be specified as an import target if Prohibit duplicate values is disabled for the key field set in the Lookup field's settings.

With the new file import method:
A Lookup field can be specified as an import target even if Prohibit duplicate values is disabled for the key field set in the Lookup field's settings.