It varied over time. Initially only anesthesiologists board cert were eligible. I believe the first exams were in 1992 and the other requirements were very soft and subject to interpretation. When that window closed then pmr started their cert exam and opened it to essentially anyone who had ever thought the word "pain" and any specialty except anesthesiology was eligible to take the little exam with soft requirements about pain experience. Neurology followed but excluded pmr and anesthesiology.