This is blank when jobs are first created, however if it is updated after that and the additional status-based rules have been configured to send emails, it will be the email address of the selected contact that will receive the email from the Notify module.
In the case below, where two email entries exist, Jim2 will choose one of them.
To be certain which one will be used, there should be only one email contact entry, however multiple email addresses can be in the same field, separated by a semi-colon () which will then email to more than one person.