This page is linked to the feature segment of AcyMailing, here you can link segment to your campaign to send more accurate emails.
Chosen lists
This part of the page is a summary of the previous step: recipients, you can see which lists you've selected and the total number of users subscribe to these lists.
Segment
This part of the page is where you can add a segment to your campaign.
There is 2 way of doing it:
Select an already existion segment that you've created in the segment menu
Create your segment for this particular campaign
If you create a segment for this campaign and you would like to use it again after you can save it by clicking on the button Save segment on the right of the section.