It's been a long time that I want to write about Ctools. I have two good reasons: They are extremely useful -God bless Earl Miles, and it is very short on documentation.
Ctools have many tools to offer and I am going to explain some of them, one at a time. This time, I want to show you how to use Dependent tool: It make form items appear and disappear based upon the selections in another item.
When using Dependent tool, you have two kind of fields:
Field that others are depended on its value can be one of these types: Radios, Checkbox and Select. Dependent fields can be anything, even markups.