Under Communicate tab, what does the message “Places may take up to 24 hours to sync with existing instances of the SDK" mean?

Since the Gimbal SDK utilizes caching when possible to increase optimization, some of the information you create and change in Gimbal Manager takes up to 24 hours to reach each client SDK.

If you do change your communication such as to add a limit and frequency to your communication, it can take up to 24 hours to see those changes reflected for all of your app users.

You can, however, uninstall and then reinstall the app so that you get the latest changes as reflected in the gimbal manager which can be useful for testing if you need to see changes reflected more rapidly though please realize that this can sometime has unintended consequences (for instance, not having your GCM tokens properly expire on Android resulting in multiple notifications for the same place arrival).