Character

The index position number is determined by the order in which the .idx index files and .cdx compound index tags are specified in USE and SET INDEX.

You can use SET INDEX, SET ORDER, and USE to specify which .idx index file or .cdx compound index tag is the master controlling index file or tag. For more information on specifying a master controlling index or tag, see SET INDEX, SET ORDER, and USE.

"0" is returned if there isn't a master controlling .cdx compound index tag or .idx index file (for example, SET ORDER TO is issued to display and access the table in natural record order).

Reference