This variable is the segment number that CMS assigns to identify a geographic market segment or subdivision of a Part D plan; the segment number allows you to determine the market area covered by the plan. The variable describes the market segment for a given month (April).
If the beneficiary did not have a Part D plan for a given month, this variable will have null/missing value for that month. If the beneficiary changed plans during the year, the value indicates market segment identifier for the final, reconciled PBP. For 2006 - 2012, this variable was always encrypted to comply with CMS privacy rules.
There are 12 monthly variables - where the 01 through 12 at the end of the variable name correspond with the month (e.g., 01 is January and 12 is December).
You need to know the Part D contract number (PTD_CNTRCT_ID_XX) and plan benefit package (PTD_PBP_ID_XX) in order to determine the geographic market areas where the particular PBP was offered. Premiums may vary by market segment.
Source: CMS Common Medicare Environment (CME)