The 2nd tab helps you to create your Distribution List (Mailing list)
The view changes according to your selection in the Campaign targeted to Dropdown list in the Campaign Def tab
If you have selected All Contacts previously then just click Create distribution list button in this tab.
If you have selected Category previously then Select a Master category in the Select Master Category list in this tab. Click the Create Distribution list button
If you have selected Group previously then Select the Group attributes and its value. Click the Create Distribution list button
If you have selected Specific Contact previously then Select the Search criteria for the contact and then Click the Create Distribution list button.
Distribution list will be created if and only any contact matches your search criteria. You can get to know from the Distribution list count.
If you are regenerating the Distribution List in Edit mode, i.e. you are editing a campaign, any previous Distribution List generated for the campaign will be over written.
The Distribution List form closes automatically after the List is created. Wait for some time to complete the Leads verification process. For this process another dialog opens within 2 seconds of closing the Distribution List Dialog.