InfoPath 2003 uses three techniques of validating values entered by a user into an InfoPath form.
A W3C XML Schema document exists or is created for each InfoPath form template. The schema document will constrain the structure of the XML data structure underlying the form template and might also define which data types are permitted for fields in the form template's data source.
JScript or VBScript code can be used in association with InfoPath's validation events to provide validation checks in addition to the constraints imposed by W3C XML Schema.
The InfoPath user interface allows validation rules that cannot be expressed in W3C XML Schema to be specified for certain types of form control.
