They are assigned if your play warrants it and you do not have one already.
If you have a recommendation from a friend, it is good to ask a specific host to serve as your host sometimes though your level of play or what you play may not be their area so they may turn you down.
Changing a host at a property who you do not like can sometimes be impossible or a problem that is why if there is someone you know about who is good, it is good to speak with them about being your host.
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