Later versions of dBase and FoxPro have added new expression functions that will not be recognized by baserunner. If you use a compound expression for a tag, seek values must match the exact format retruned by the tag expression. Padding will not work on seeks using compound expressions. The ability to add a filter to an index tag is quite powerful. When a filter is added to a tag, the index file keeps track of the records that currently match that expression, and simple switching to that tag will give those results.
Unique tags are especially useful when creating query menus.