To be even more precise: filters

To be even more precise: filters In OBS, you can add an "image mask" filter. This allows you to import a shape a circle, a rectangle, etc. to crop exactly what you want to show.

Concrete example Want to show a single exercise from a PDF?

Just capture the PDF window. Crop around the exercise. Result: students won't see your menu bar or notifications.

In short To show a single area in OBS: use Screenshot or Window Capture, then crop. It's simple and avoids distracting students.