The variable is the Part D plan benefit package (PBP) for the beneficiary’s Part D plan for a given month (May). CMS assigns an identifier to each PBP within a contract that a Part D plan sponsor has with CMS.
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 the final, reconciled PBP number.
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 both the Part D contract number (PTD_CNTRCT_ID_XX) and plan benefit package in order to identify the specific plan in which a beneficiary was enrolled.
Source: CMS Common Medicare Environment (CME)