This help book is a sub-book to the Indexing and sorting dBASE tables book covers the following information:

dBASE Plus lets you create primary and secondary indexes for any table type that supports them.

A primary index is the main index in a table.  For DBF tables, only level 7 tables support primary indexes; any expression index may be created as the primary index.  For all other table types, the primary index consists of one or more consecutive fields, starting with the first field in the table.

A secondary index is supplemental to the primary index in a table.