Final answer:
The CMS-assigned UPIN consisted of 6 alphanumeric characters, which was used for identifying physicians in Medicare claims before the adoption of the NPI system.
Step-by-step explanation:
The CMS-assigned Unique Physician Identification Number (UPIN) previously had 6 alphanumeric characters. This number was used to identify physicians in Medicare claims prior to being replaced by the National Provider Identifier (NPI) system. UPINs were designed to be easily manageable, hence their relatively short length of only 6 characters.