To add a comment on your test fit navigate to the top left hand side of the interactive test fit.
Once you click on Comments the right hand panel will change to accommodate this functionality, where you will be able to write multiple comments on your testfit. In the box provided, type your comment and once completed, click on the + sign next to "add new comment" and place your comment on your plan in your area of interest.
Click and drag on your plan to highlight (create a box) if you'd like to reference a specific area.
To delete comments, while in comments mode, click on the top right of the comments box and press delete.
To hide your comments you can navigate to the layers button and uncheck the comments box: