Please explain difference between Core and Extended

What is the difference between “core” and “extended” levels? (A definition for each type would be helpful here.) Is “core” required? Do you generate an error when core fields are missing?

No. 2 of 16. This question was asked by a new ECS user, who is familiar with mapping IT events to data models and use cases in other schemas. These questions are being posted as a GitHub issue, because a) they may offer valuable insights. b) we expect that many new users will have similar questions.