A unique number assigned to a list member, usually by the email-broadcast software, and used to track member behavior (clicks, subscribes, unsubscribe) or to identify the member to track email delivery.

3 Quick And Easy Ways To Build A Profitable Opt-In List
You finally realize that you need a good opt-in list. After reading countless articles and sought expert advices and have read many success stories of people creating a small fortune with opt-in lists you finally decide to have one of your own.