By default, anything other than the email address is considered a custom field in ListWise HQ. ListWise HQ, currently only supports a maximum of 12 custom fields per list imported for cleaning.
After you import the list, if there are custom fields, you will be given the chance to specify which custom fields you wish to carry forward for cleaning.
While ListWise HQ does nothing with your custom fields, it keeps it so when the email addresses are cleaned, the list we provide you contains the same custom fields you uploaded, associated with the right email record.
If you import a list that has more than 12 custom fields then ListWise HQ will still clean the list, but will only return 12 columns back in the cleaned list.