AM Law 100 refers to an annual ranking by the American Lawyer publication of the highest revenue grossing law firms. This ranking tells you as much about law firm sizes as anything else, but it is a usual reference point for “large/major” firms. IMO the list that tells you more about the health of a law firm’s business is Revenue per Lawyer. The AM law lists are behind a paywall, but this article provides the top 10. There is only an overlap of 2 firms in the top 10. A Deep Dive Into The 2021 Am Law 100 Rankings - Above the Law
Not sure about EP, but in the context of the post, I am guessing Existing Practitioner?