叫做虚拟变量，偶尔也叫做“哑变量”，
It is whether 0 or 1.
Why we need to set a baseline group?
It is so called the Dummy Variable trap. We need to avoid the perfect co-linearity. So we have to exclude one of the dummy variables. Different pairs of dummy variables included in the equation imply different benchmark/reference category.
Another way to avoid the perfect co-linearity is to include all the dummy variables but exclude the intercept term.
Reference
计量经济学 | 虚拟变量 - 小喵的文章 - 知乎 https://zhuanlan.zhihu.com/p/359928473