According to The Oregonian, a vertical slit shaved off the eyebrow is a gang symbol indicating membership. It is sometimes referred to as "carved."
In some cases, especially in schools, students with "carved" eyebrows are told to pencil them in. Some schools do not ask their students to do this unless advised by law enforcement agencies.
Shaving a line in the eyebrow might be likened to retaining price tags and size stickers on hats. This can also be viewed as an indication of gang status.
According to The Oregonian's source, a shaved line in the eyebrow is meant to impress rather than to intimidate.