I see the words "CRPS Expert" on several different posts and it got me thinking. What makes someone an expert?

Does a physician have to be in a certain field? Are there any criteria one should look for when finding an expert? Are some of these doctors declaring themselves experts? Does one have to be published? If so, how many articles?

When one is looking for an expert is there a website that will list the doctors that are considered an expert?

Sorry for all of the questions but I am curious to see what others think.