You can find a tutorial and downloadable examples (with the correct use of displayParam as well) in this lesson of the Learning section. The displayParam is not always necessarily the edited parameter. It can be any other parameter represented, but the connection between the edited and represented ones should be handled in the parameter script of the object. Check out Central editing example AC19.gsm in the downloadable lcf for example codes.
Dominika Bobály
Product Manager
GRAPHISOFT SE