There are only two topics available that start with "losant". They are:
The first one allows the publishing of Device State. The second one is typically used to subscribe to Device Commands.
Any topic that starts with "losant" is a special topic that is used for one of the above purposes. If you attempt to subscribe to any other topic that starts with "losant", your device will be disconnected.
If you want to use custom "xyztopic", you can do so, but it can't start with "losant". You can publish and subscribe to any other topics. For example, you could use "my/custom/xyztopic" and that would work just fine.