How to list all the namespaces?
There are only three namespaces available: system, secure and global.

The settings command prints this information if you execute it without parameter:

adb shell settings

usage:  settings [--user &lt;USER_ID&gt; | current] [--cm] get namespace key
        settings [--user &lt;USER_ID&gt; | current] [--cm] put namespace key value
        settings [--user &lt;USER_ID&gt; | current] [--cm] delete namespace key
        settings [--user &lt;USER_ID&gt; | current] [--cm] list namespace

'namespace' is one of {system, secure, global}, case-insensitive
If '--user &lt;USER_ID&gt; | current' is not given, the operations are performed on the system user.
If '--cm' is given, the operations are performed on the CMSettings provider.



  'namespace' is one of {system, secure, global}, case-insensitive


Alternatively if you don't belive the settings help page you can see it in the AOSP source code. The settings command is implemented in the Java class com.android.commands.settings.SettingsCmd where you can see that exactly those three namespaces are accepted.