The answer to your question is on the cloud dashboard.
This is the permissions for public database your looking at, you can make it writable as I have here for an authenticated account, so somebody logged in with their iCloud account, but there is no option to make it writable anonymously.
But I can see the confusion here, the database belongs to the app, not the user and you can make the public part writable by anybody as I have here. So in your example your maps app it would write to its public database so that other users would have access to it.
The caveat you need take care of here is the fact that the quota for the public database goes with the app's owner, you. So you don't want to put too much data there or indeed give even authenticated users the ability to upload heavy objects, cause if you do you may come unstuck with the finances. You have a quota, which gets bigger the more users you bring on board, but how that works out in the real world is a challenge to manage.
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