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  Compound (.CDX/.NSX) Indexes:

  A .CDX or .NSX file is a Compound Index - an index file contains multiple
  indexes (called tags).  Compound indexes allow you to have several
  indexes available to your application while only using one file handle.

  A tag within a compound index is essentially identical to an individual
  index file and supports all of the same features.

  Once a compound index is opened, all the tags contained in the file are
  automatically updated as the records are changed.  The first tag in the
  compound index (in order of creation) is by default the active tag when
  the index is opened.

  You can have several compound indexes for a single database file, giving
  you the ability to go well beyond Clipper's normal 15 index limit.  A
  compound .CDX index can have as many as 99 tags, but the practical limit
  is around 50.  The tag limit for .NSX indexes is 50.

  Structural (Auto-Open) Indexes:

  A "structural" .CDX or .NSX (equivalent to dBASE IV's Production .MDX) is
  a compound index that is automatically opened whenever its database file
  is opened.  The structural index file must have the same base name as the
  database file, and the extension must be ".CDX" under SIXCDX and ".NSX"
  under SIXNSX.