What an Expression Box Is
Chapter 4, “InfoPath Form Controls,” describes almost all the InfoPath 2003 form controls. The expression box control gets its own chapter because it has some characteristics that are different from those of other controls.
In particular, an expression box control requires you to use XPath to create XPath expressions that are associated with the control. This technique allows you to display in a form view values that aren't entered by the user, but that are derived in some way from data the user enters into the view.
For example, you can automatically number an item by counting its position in an XPath node-set. Alternatively, you can sum the numbers in a column of numbers or find the average of a group of numbers.
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