I understand your explanations as to in which circumstances the boxes are checked or unchecked.
What I had found, and this is why I made the post, was that discussions to which I subscribed using the "Subscribe" icon under a post were NOT listed in my "Discussions subscribed to" section of my posting history.
And in case you think I am imagining this, since yesterday a discussion to which I was most definitely subscribed: "Feminism," has been moved from the "Discussions Subscribed To" area back to " Discussions Participated In? area of ?My Discussions.?
I do not necessarily wish to receive emails every four hours of new activity, so I have unchecked the box below this post and will wait and see if I remain subscribed to this discussion.