You have a few options to identify churning users.
1. Smart Segments
Smart Segments are automatically generated by Batch from the moment the SDK is implemented in your app. They allow you to take action on your userbase in a meaningful manner, based on their engagement level.
Dormant users were 'engaged' at one point, but have not launched your app for a long time. They're basically not using it any more, but they still remember it and probably why they installed it.
To target them, just select the "Dormant" segment in the targeting section of the campaign creation form.
2. Last visit date attribute
Maybe smart segmentation is not precise enough for your CRM scenarios. Batch's SDK automatically detects the last visit date of your users in your app. If you want more control in your targeting, you can take advantage of this attribute to create segments as "Users who did not visited the app since N days"
3. What should I send to these users?
Give these users a good reason to start using your app again such as new features, a special offer or anything showing that you've improved since their last session.
A good idea can be to complete the initial targeting (Smart segmentation or based on the last visit date) with other criteria as the use of a specific feature, or consumption of a specific content. It will allow you to create truly personalized campaigns, relying on your users' preferences and interests.