Predicates allow you to specify expressions to match against pre-defined variables to choose which marshaller will get used for a given marshall/demarshall request.
Condition here, is a TALES expression to be evaluated when deciding if the PropertySheet should be applied to the given object. If you used Page Templates before, those expressions are very similar to Page Template expressions. The variables available for building the expression are: object_url, object, nothing, user, modules, request, mode, filename*, content_type*, data*
* Note: those may only be available during demarshalling and are not guaranteed to have valid values, as they may depend on the value the client sends.
Examples:
python: mode == 'marshall' and obj.portal_type == 'News Item'
python: mode == 'demarshall' and content_type == 'text/xml'