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  What is an index SCOPE?:

  An index scope is a specific range of keys within an index file set using
  the SET SCOPE command or Sx_SetScope() function.  The SIx Driver's SET
  SCOPE commands and functions let you set high and low key limits so that
  you see only the range of records that you want.  The index scoping feature
  is similar to SET FILTER, except that it works with indexes and is MUCH
  faster!

    NOTE: For those situations where SET SCOPE cannot be used, such as when
          a compound condition is needed, our Mach SIx query optimizer can
          optimize SET FILTER with nearly the speed of SET SCOPE.

  Also, you can set a scope for each index or tag, and the scope remains
  active as long as the index is open.  One limitation that index scopes have
  is that the specified top and bottom boundaries must be constant values.
  If an expression or UDF is supplied as the scope value, then the return
  value will be used for the scope value.  This eliminates the slow-down from
  constant re-evaluation of a filter expression.