See: http://requesttracker.wikia.com/wiki/ToggleCFs
“We had a need to show/hide custom fields depending on the
value chosen for a certain “control” field. That is, if the control
field had a certain value, we wanted to show a certain set of custom
fields for editing and display. If the control field did not have that
value, we wanted to hide those custom fields entirely. This could
probably have been done easier in Javascript, but nobody on our team
knows Javascript.”