The Segment Tab is where you control which of your uses you would like the campaign to apply to.
The Segment Tab initially has two checkboxes visible. Broadcast to all users, and target users.
The default is broadcast to all users. What this means is that the notification you are creating will be sent to all users with your app who have registered with the platform and accepted notifications.
However, if you would prefer to target the audience of your notification, then you select target users as I have done in the example above. This will open up a new area underneath, where you can select your campaign conditions with the xtremepush segmentation engine.
The example above shows a segment which targets Irish users who have not opened up the app in the last three days.
The segmentation engine has a large number of options and further information can be found here. If you have user id in xtremepush that is matched with one on your backend system then you can also upload a segment of users created on that system from a csv file more details here