This would be really great to post in some obvious spot (like the documentation???) I’ve just spent hours looking for this. If you search on the particle.io site using the search bars on various pages, they never show this. Only a google search of “site:particle.io repeater” finally finds this one thread.
Especially since the blog post describing the Xenon says
"But this raises a big question, which of the two roles — repeater or endpoint — should a Xenon take? "
That sure makes it sound like it’s something the user needs to set, or that it’s at least exposed to the user…