I try to understand Chinese 9 dash line but I can not. Maybe only Chinese are smart enough to understand
China claims that they own the sea inside 9 dash line from Han dynasty but just 200 years ago, when we are at the sea, we can not know exactly where we are. So even we are right above the 9 dash line, we also don't know. In the ground, people can use some marks to specify the borderline but in the sea, we cannot without modern technology. However China claimed based on Historic evidences from two thousands years ago.
China also don't explain why inside the 9 dash line, there were Chinese Historic activities but outside the 9 dash line there were not. Coordinates of each dot was also not clear.
And importantly just 200 years ago, Even If Chinese Emperor went to the sea, He also didn't know where are the positions of that imagined borderline. Even Zheng Hua crossed over that imagined borderline, he also can not recognize.
Remember that we only know where we are in the sea to respect the EEZ with the help of modern GPS. That's why there was no concept of sea borderline in the past.
Only Chinese think they can specify a borderline in the sea from hundreds years ago.