You can choose between two suitable fields: Customer ID or Email. The app automatically differentiates between two main fields. If one of the fields is empty and another is filled, the information will be taken from the filled one. To make it work properly, ensure your CSV file has both fields (Email, Customer ID) and corresponds to this format. Even if you don't have any Customer ID fields, you must include at least a column with its name.
NOTE: Rewards are not issued for the imported VIP Tiers.