What data that should be included in your import file depends on what data you wish to either be presented in the your sendouts or what data should be used for segmentation.
The more data you import, the more segmentation can be made but select relevant data that will help you to create smart communication. By importing non-relevant data, it may get harder to keep track of your core data and may also cause unnecessary import and export time.
One requirement is that the recipients in your import file are identified by an ID, as OriginalID. This can be a primary key from a database or anything else that uniquely identify each recipient. The result of this is that the simplest import file must at least contain two columns (OriginalID and email address/phone number).
When no suitable ID for recipients can be found you can choose to identify recipients by their email address or phone number.
PLEASE NOTE: That this is an inferior method and by doing so recipients that change their email address or phone number will be treated as new recipients in the system.