The boundary files in the ZIP5 folder contain US 5-digit ZIP codes.

Some ZIP codes are only used for post office boxes - there is no
delivery area associated with them.  These PO Box only ZIPs are
represented by point objects in the files that end with "-p".

The other ZIPs which do have a delivery area are represented by area
objects in the files that end in "-a".

There are two files per state, one with the area objects and one
with the point objects.  Taken together, the area and point files
provide a nearly complete coverage of US 5-digit ZIP codes.

Besides these boundary files, there is another file included with
MapViewer that contains ZIP code location information.  It is
named ExtendedZIP5.csv and is located in the MapViewer 6 installation
folder, normally C:\Program Files\Golden Software\MapViewer 6\.

This file is used internally by MapViewer when making a ZIP-based
pin map.  In addition, it can also be used for a coordinate-based pin
map to create point objects for all of the ZIPs it contains.

The ExtendedZIP5.csv file contains approximately 700 more ZIP codes
than the area and point boundary files, out of approximately
43,000 total 5-digit ZIPs in the US.