Yesterday I had a question from one of my customers asking why, after their laptop upgrade, OneNote always opened as a full screen on the monitor, and new applications would launch behind the OneNote screen. The only way they could see the other application was to minimize OneNote. I was completely unaware that there is an Always on Top setting in OneNote! Here’s where that is…
You can find this setting in OneNote under View > Always on Top. It’s a toggle setting where clicking it turns it on or off:

If you have this turned on, the OneNote client will show up full screen and will remain on top regardless of what else you launch on that monitor. If you have it turned off, then OneNote acts like a regular application, and new items launched on that monitor will overlay the OneNote screen.
I see this as one of those tips where you either think “oh, that’s a GREAT feature!” or “I’m SO glad I know how to turn that off now!” 🙂