The IDs in the Census2000 data set are essentially zip codes, but not exactly. 
These are technically called ZCTAs and were set up by the US Census Bureau. On
the Census Bureau's web page you can find answers to questions about these ZCTA 
codes and the following excerpt deals with the XX and HH codes that have the 
extremely low population densities and household incomes. 


  11. What do the ZCTAs with an "XX" or "HH"suffix represent?
      The three-digit ZCTA code and an "XX" suffix are applied 
      to a large land area (generally larger than 25 square miles) 
      where we had insufficient information to determine the 
      five-digit codes. The USPS may not provide five-digit ZIP 
      Code delivery service in this area. These are generally rural 
      areas with little settlement; for example, parks, forest lands, 
      and desert and mountainous areas. Smaller areas were generally 
      absorbed into adjacent five-digit ZCTAs, but for larger areas, 
      we did not want to overextend the surrounding five-digit ZCTAs.

      The requirements for ZCTA delineation state that all census 
      blocks receive a ZCTA code. Because large water features may 
      border many five-digit ZCTAs, it would be difficult to divide 
      up the water features and assign the parts to the five-digit 
      ZCTAs on the land. Furthermore, any attempt to add these water 
      features to land-based ZCTAs, would greatly distort the size 
      and shape of the land ZCTAs, particularly in coastal areas. 
      To avoid these problems, such water features receive ZCTA codes 
      ending in "HH" (special code for hydrographic features).