Forms are usually defined in a declarative style, with form fields listed as class fields. However, sometimes we do not know the number or type of these fields in advance. This calls for the form to be dynamically generated. This pattern is sometimes called dynamic form or runtime form generation.

Imagine a passenger check-in system for a flight from an airport. The system allows for the upgrade of economy-class tickets to first class. If there are any first-class seats left, then it should show an additional option to the user, asking whether they would like to upgrade to first class. However, this optional field cannot be declared since it will not be shown to all users. Such dynamic forms can be handled by this pattern.