The commenting panel is located on the right side panel on Jupyter's main area.
When opened for the first time it will ask for you GitHub username to know who is commenting. This uses the public GitHub API to retrieve your name and profile image.
Once logged in, you are able to do a variety of things.
Once signed in, you can create a comment thread by clicking on New Comment Thread. Once the thread is created others can reply to your thread.
Threads can be resolved by clicking on the Resolve button. A thread can be re-opened by clicking on the Re-open button.
Clicking on a comment thread will expand the conversation and enable the option to edit by hovering over a comment and clicking on the edit button.
In the same way as editing, once a thread is expanded you can delete a comment with the delete button.
By using the drop-down on the top, you can sort comment threads. The show resolved check-box can be used to filter resolved threads.
To comment on a text file, you can either select exactly what you want to comment on, right click, and select the option Create New Comment, then go about creating the thread. Or you can just right click the line you want to comment on, and the entire line will be marked when Create New Comment is selected.
To focus an indicator you can either click on the comment thread in the commenting panel, or you can click on the indicator in the text editor to focus the related thread.
Comments can be created on notebook cells.
To make a comment on a rendered cell, like a Markdown cell you can click into the edit view and make a comment.
Currently, comments are saved in a generated file named comments.db. This file saves all your comments and indicators. The file can be shared and others can copy it to their file tree.