[Zope] How to prime a selection?

Hi all I'm making a document class for a warehouse of papers. It's based on ZCatalogAware and ObjectManager (to wrap mainly PDFs and DOCs), and has fields: title (string type) origin (string type) format No value for format_value. type No value for type_value. "format" and "type" are selections -- 1. where do I supply their initial values (filling in 'format_value' and 'type_value')? 2. is it possible to let a field offer presets and previously entered values, but to allow users to add new ones, which are remembered and offered to subsequent users? A RTFM-type answer to question 1. is perfectly OK, though a page reference would be sorely appreciated. -- jean