Where do I find rules governing Personal Messages this site?

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Is it "no holds barred", that is, is it "anything goes without penalty" for the content and style of personal messages?

Are there a set of rules posted about the conduct members of this site must adhere to when it comes to privately messaging each other?

If there is a set of rules posted, please publish either a link to it or a way to find it on the site.

Thanks.