This variable is the provider identification number.
The first two digits indicate the state where the provider is located, using the SSA state codes; the middle two characters indicate the type of provider; and the last two digits are used as a counter for the number of providers within that state and type of provider (i.e., this is a unique but not necessarily sequential number).
Refer to CCW Technical Guidance document: "Getting Started with Medicare Data" for additional information regarding service setting classifications.
If you want additional information about the institutional provider, the quarterly CMS Provider of Services (POS) file contains dozens of variables that describe the characteristics of the provider. This file is updated quarterly, and effective May 2014 is available for free online from the CMS website (2005-current).