The average quantity of items being purchased per transaction.
Most retailers would want their customers to buy higher quantities of a single product and would use multiple measures to encourage this behaviour. A business that can sustainably increase its units per transaction will increase the ROI per cost of attracting a single customer. Units per transaction is related to the customers' purchasing pattern, and analyzing this can lead to opportunities to increase sales per customer.
Specific products that are occasionally purchased in larger quantities suggests that the product caters to multiple use cases, or multiple demographics. Studying this can help you cater to these new opportunities for up-selling or cross-selling.