Category III CPT codes are temporary codes that describe emerging technology, services and procedures. The primary purpose of these codes is to allow for data collection, which in turn provides information for evaluating the effectiveness of new technologies and the formation of public and private policy.
In other words, category III codes give insurers and government policy-makers a way to track the effectiveness and rate-of-use of as-yet-unproven technologies, which could affect future coverage decisions. Such data collection is not possible when coders simply use the unlisted-procedure codes.
The designation of a category III code is an important first step in wider adoption of new technology and the eventual creation of a category I CPT code to describe the service (although this doesn't happen in every case). Generally, a category III code will be archived after five years unless the AMA has already created a category I code for the service.