It is a distribution list that you can use for Report Schedules.
For example, if you have a group of promoters or producers who regularly receive the same type of report, then you can group their email addresses together in Document Delivery Method.
This means that you can simply select this group when creating a Report Schedule, rather than typing all the individual email addresses out.
There are two types of Document Delivery Methods you can create:
Email - where you can add the recipient email addresses that will be grouped together for your Report Schedule.
SFTP - Secure File Transfer Protocol. This guarantees a secure way to send files and is handy if you're running a data warehouse.