If you just leave the field blank, the import will skip that field and not change the data that exists in that field. If you try to import data into a text field and that data is longer than what the text field can accept, your data will be imported but the excess data will be truncated. For example, the employee name fields are limited to 50 characters. If you try to import a name that exceeds 50 characters, the excess characters will get cut off.
This import is for updating vouchers it will not create vouchers. This import cannot be “undone”. Once the import updates the data, there is no way to automatically reverse what you just did. But you can access the vouchers and edit them to make corrections.