Demand fields allow you to define field logic between two fields at a time. This is useful when two fields are interdependent, for example, when they must be filled in as a pair, when two fields must not coincide, or when one field is to receive a complement if another lacks a value.
Demand fields are specified in the Field dialog for image, mark, character, object and barcode fields. You can define up to three demand fields for any current field definition of those types.
While demand conditions can be invaluable in certain situations, you must be careful to use them correctly, keeping their limitations in mind.
For example, the fact that Verify shows you an empty field that a respondent should have filled in does not solve the problem that it is not filled in. Usually you cannot simply guess at appropriate contents.
While it can be useful if FORMS draws your attention to fields that are not properly filled in, often demand fields simply cause additional work during verification. Before you set up demand conditions, have a look at the Demand field conditions and actions dialog and consider exactly what actions you want to take during the verification process if the conditions are not met.