I didn't know that and it certainly wasn't at the front of their foreign policy until the CCP revitalised it recently. Any way you look at it the 9 dash line is a blatant and greedy attempt at increasing territory and control at the expense of their neighbours. It extends thousands of kilometres from China right up to less than 200km (the internationally recognised EEZ for all countries) from Vietnam, Malaysia, Brunei and the Philippines. Areas inside the 9 dash line are closer to Laos, Thailand, Cambodia, Indonesia and some other countries than China even though those countries don't have a sea border with the area. If the CCP wasn't so serious about it you could just call the 9 dash line an ambit claim but as it stands it is a threat to the stability of the whole region.
