When creating a data extension aimed at targeting prospects or recent purchasers who are not loyal members for Northern Trail Outfitters, the associate needs to define the "Data Type" attribute appropriately. The "Data Type" attribute helps in specifying the kind of data each column in the data extension will hold, such as text, number, date, etc. In this context, a specific data type, possibly a Boolean (true/false) or a text field, could be used to indicate whether a customer is a loyalty member or not.

By properly setting up the data type for a loyalty member indicator field within the data extension, the marketing team can segment their audience effectively, ensuring that communications are directed appropriately to prospects or recent purchasers who are not yet part of the loyalty program.

References: The Salesforce Marketing Cloud documentation on data extensions includes guidance on defining data types for each field in a data extension, which is crucial for accurate data management and segmentation.