"Screen region" is XSplit's equivalent of "Monitor Capture" in OBS. As suggested, use Window Capture or Game Capture instead. Any guides that mention "Screen region" were probably written for XSplit in a time when "Game Source" (AKA "Game Capture" in OBS) didn't exist. Or it was written because when Game Source was implemented, it was a paid feature (it's free in OBS).
Long story short, the guides are outdated and you should be using Game Capture or Window Capture when streaming games. With Game Capture, the game can be in fullscreen or windowed mode. With Window Capture, the game HAS to be in windowed mode and you should leave Aero enabled.