Major incidents vs. high priority users

Answer:
Major incidents have separate procedure due to their scope and (usually) emergency situation. You can find more details in following article: "Major Incident Management – when the going gets tough…" https://advisera.com/20000academy/knowledgebase/major-incident-management-going-gets-tough/
However, some companies define high-priority users (as you mentioned - P1 users), like e.g. Board of Management, director or the owner of the company. Their incidents are resolved before other incidents of the same priority. But, if there is an major incident and an incident from P1 user, then major incident should be resolved first (usually, P1 users are affected with major incident anyway).