On any PR, go to GitHub to the changed files tab of that PR and add some review comment to some line of code, or suggest a change.
You can see it somehow in the issue tracker, too.
You can see this kind of comment only on GitHub.
It can happen that testers waste time for testing a PR because they can't see that there is a review comment telling about a mistake in the PR or a proposed change.
We’ve been storing this data for a while #982 purely a matter to find a nice way to render it in the UI