VSTS/PR - Required Reviewers when hovering over the reviewer, have the custom message field (if populated) display for the reviewer.
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The Custom Messages field shows up in the comment feed, only to be scrolled down way to the bottom and never seen again (Then users ask us why there's a required reviewer and how they fix it)
We provide custom messages if the required reviewer being added is because of an issue in their branch, disallowed file/type/location or a specific team is needed for signoff and an email to reach them.
The Custom Message should be more apparent and up front to users so the meaning doesn't get lost in the comment stream. Pinning and/or Showing the message on hover would definitely help make it more visible.