So, I need to emit particles only from the yellow area. The particles are a group of tiny rocks, they need to be randomly emitted. I tried using the grease pencil to select certain areas (creating a bezier curve from pencil), but this would get time-consuming really fast, and besides, I could not get it to work. Note: I asked a previous question to create the cube, here was the methodology: Previous Question
EDIT 1: Here's the file link: Link.