You can select all the lists the campaign will be sent to.
Search : This input allow you to search a list, it's refreshing alone after you search something.
Select all : Add all lists to the selected lists.
Unselect all: Move back all selected lists to the available ones.
Campaign will be sent to a total of : the number of users who will receive this email. Each user is only counted once, even if subscribed to several selected lists. You can see on the picture before that the number of recipients is lower than the sum of subscribers from selected lists (one is subscribed to both lists)