ScriptableFields is a collection of fields  dealing with logic 
instead of content. This is kind of evil and if you use those 
fields be sure you really want it this way!

The bundle contains:

TALESField -- a StringField like field you store a TALES expression
              in. It gets evaluated on 'get' and the result is
              returned.

PythonField -- a TextArea like field where you can store a
              whole Script (Python) in. On 'get' its evaluated  and
              the result is returned.

DTMLField and ZPTField -- both fields working like PythonScriptField
              but the given text is evaluated as DTML or Zope Page-
              Template.