First of all, please make sure to read "How to start as a Player" first.

I have multiple lands. How can I claim for a specific one?

Answer can be found here: Link

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I have multiple lands. How can I select the land I want to use for my commands that I don't have to specify it in the commands every time.

Same as the previous question: Link

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I'm part of multiple lands. At least two of them are now engaged in a war. Which team do I join?

Defending lands are prioritized and the team is calculated once and stays the same during the whole duration of the war.