What does the warning signal at the beginning of E6 mean?

the code names of China people's police are as follows: a: immigration management, b: exit and entry management, c: frontier defense management, d: police command and tactics, e: security science, n: public security information science, o: network security and law enforcement, p: data policing technology, t: public security and political work, w: foreign-related policing, x: fire fighting, y: graduate student.