Pull Request: Assignees vs Reviewers?

So new user here, I am finding that there is a lack of documentation depth on how to do things or I am failing to find it.

In creating a pull request, there is a button to add assignees which Interpreted to mean reviewers. After creating the pull request, and I look at the pull request, it says no reviewers but does have the assignees

I can see after I create the pull request I can add reviewers.

What is the semantical difference between an assignee and a reviewer?

Hm, maybe assignees should work on this pull request with poster together. But at the moment, reviewers could do a code review and approve or reject the pull request.