aCalendar is a user interface to the local calendar storage of your Android phone. That's why all normal calendar apps (like e.g. Samsung calendar app) show the same data (only some apps, like the MS Outlook app provide their own proprietary sync and do not integrate into the Android calendar system).
Under manage calendars you are able to create new calendars:
- Local calendars, which are only shown and saved on your Android phone. These calendars should be manually backuped by yourself, as you risk data loss when your device breaks or get lost. Please see: Using local calendars
- Cloud calendars, which are automatically synchronized between your chosen cloud server and the Android calendar storage. For cloud calendars you need to have an account. Most popular is Google, but it works with any cloud service which is able to sync your data to the local calendar storage of your Android phone.
aCalendar itself saves no user data and provides no own synchronization with a cloud service or an own backup solution.
For problems with calendar synchronization please see: Troubleshooting Cloud Synchronization