The Preferred field is commonly enabled to designate Customers as part of a group (of Preferred Customers) and may be used when filtering on reports or when exporting the Customer database.
Enabling the Preferred flag has no affect on Customer pricing or on how their Sales Transactions are completed.
The Preferred field may also be used in Label Manager when printing labels. Mailing labels for only Preferred customers may be printed when offering coupons or promotional sales events.
See:
Customer Database - Main Screen Fields