[Zope] Selection Box

I have searched most reference material with no avail to try to find out how to set up a simple selection box. I have a ZClass called LinkClassification to which I have attached a property link_class which is assigned to value link_class_data. My question is where do I create the variable link_class_data? I think it should be as another property in the same directory. When I do this the zope management screen hangs. I presume this is because I did not define the link_class_data property correctly. What are the valid types allowed for a selection box - strings,text,etc.? Does someone have the syntax for how the selection box is assigned a variable? Or is there an example somewhere I can reference? Thanks, John