For example, the teacher corrects homework and circles the key points in the book. At this time, the red pen is mainly used to distinguish between correction and original writing.
Writing in red ink is disrespectful to readers. In ancient times, writing with red pen was called Zhu Pi, which was used by superiors to subordinates. If we write letters in red, it basically means breaking up with our friends. So generally don't write with a red pen.