There's a pretty big grey area, isn't there? My husband wants to know who else I am involved with, so he can avoid them. That's pretty much all he wants to know. I'm sticking to the boundaries he laid out (no PIV or oral sex) and he's trusting me to do that. He has no interest in hearing about what happens within those boundaries.
I do let him know when I am going to be with C (usually a necessity due to child care) but I don't give details. The other day I told him I was meeting C for breakfast, but not that the "breakfast" C had waiting for me consisted only of raspberries and chocolate syrup in a hotel room, with no dishes.
Since all of this is still new and evolving for us, I have had lots of conversations with C about what my husband says he wants or feels. (The boundaries have shifted back and forth several times in several months.) So even though there has never been a direct acknowledgement between them, each knows that the other knows what is what.
I do sometimes wish my husband was the sort of person who'd get excited by the juicy details, or even just the sweet and touching details, of my relationship with C, because I have no one I can talk to about it when I'm bubbling over with joy. (Even L, my lifelong confidante, gets too jealous.) But I know it has been a stretch for my husband to get as comfortable as he is with where we are now, and I love and respect him so much for it. Ours is not so much "don't ask don't tell" as "I would tell if only you would ask". I don't have any sense that things would blow up if he ever did ask for details, since he seems to be imagining more or less what happens anyway (usually more, I've found).