Luke Wroblewski has a great article on the various approaches for selection-dependent inputs. He clarifies what he means by selection-dependent inputs right in the beginning:
Selection-dependent inputs are, in essence, a pretty simple concept: Once a user initially makes a selection from one or more options in a form, the user must provide additional input related to the selected option before submitting the form.
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It’s worth pointing out that, in most cases, users cannot submit a form with selection-dependent inputs until they fill in the additional fields. In other words, selection-dependent inputs introduce additional requirements to forms.
He goes on to look at various solutions and their pros and cons. Forms are the most vulnerable places to ignore usability and introduce complexity. Ironically they are also the places where the focus on these should be the highest because forms are one of the most interactive elements. It is not because that the process of the information entry is complex, it is because the previous input has to be considered for future inputs. And all this while, the bigger picture or the context should not be lost. Read the article and the comments, there is a lot of meat there too.
