The NPL guides the federal government in determining which sites should be investigated. It is updated on a regular basis.
The EPA designates several NPL statuses for Superfund sites: Proposed, Final, Deleted, Withdrawn, and Removed. Other Superfund sites are not on the NPL. TOXMAP contains data only on sites designated as Final (currently on the final NPL), Proposed (proposed for the NPL), and Deleted (deleted from the final NPL). These three statuses are collectively referred to by the EPA as the "NPL Dataset".
For details on the NPL site listing process, see http://www.epa.gov/superfund/sites/npl/npl_hrs.htm.