Contrary to common understanding, air intercepts aren’t rare; nor are they prohibited. Military aircraft intercept for many reasons, including to identify, verify or escort another country’s aircraft.
Aircraft flying in international airspace are obligated to operate with due regard for freedoms afforded to all states by international law (such as freedom of overflight, as outlined in UNCLOS). Beyond that requirement, there are no established international laws regarding aerial encounters. However, there are norms and standards that help to ensure safety.
The International Civil Aviation Organization (ICAO), a specialised agency of the UN, was established in 1947 for that specific reason. With a membership of 190 of the 193 UN member nations (including the PRC), the ICAO codifies principles of international air navigation and establishes standards and recommended practices.
Military aircraft that encounter each other should operate according to ICAO guidance. Safe intercepts are typically characterised by controlled closure rates and predictable behaviour. Professionalism is characterised by proper airmanship and non-provoking manoeuvres, without any overtly aggressive actions, words or gestures.