The reason why the eight characters are divided into original qi and residual qi is because the qi in the earthly branches is mixed and contains multiple heavenly stems.
The original qi means that there is a hidden stem in the earthly branch, which is the five elements to which the earthly branch belongs, so it is called the original qi. This family, this line, the same line of work means.
The remaining qi is the heavenly stem that contains other five elements in addition to the original qi in this earthly branch, so it is called the remaining qi.