This page is linked to the feature segment of AcyMailing, here you can link segment to your campaign to send more accurate emails.
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.
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.