The primary concept is that items are shown / hidden depending on the onChange options of items selected. [[JavaScript_in_BEJavaScript_in_bxp]] provides an introduction to putting JavaScript into the form.
The primary JavaScript function for this is : [[JavaScript_Show_/_Hide_elements]] Detail of how this can be used is available in [[JavaScript_in_BEJavaScript_in_bxp#Showing_.2F_Hiding_Questions]]