This variable is the unique number CMS assigns to each contract that a plan has with CMS.
This field is a key that links the plan sponsor's contract and plan identifiers.
You need to know both the contract number and plan benefit package identification number (variable called PLAN_ID) in order to identify the specific plan benefit package offered to beneficiaries.
This variable corresponds with the CONTRACT_NAME variable, which is the contract name that corresponds with this number.
The CCW constructs the Plan Characteristics File from information submitted by plan sponsors to CMS’s Health Plan Management System (HPMS).
Source: CMS (HPMS Files)