What criteria are used to classify items as Rare, Scarce, etc.?
These ratings reflect current and recent availability. The estimates are a compendium of what shows up on the online auction systems, in the .audio newsgroups for sale, and so on. The ratings are not related to the number made, or if they are, it is only through the current apparent scarcity of an item, meaning that if there were few made, we (naturally) see few for sale now. If something begins showing up, the rating can be changed.