[Zope] the misc_ dictionary

Hi all, I'm a little bit confused on how to use the misc_ dictionary to define product resources. I've looked into other products such as ZCatalog ZGadflyDA and SiteAccess but it's use is not so clear for me especially because most products only need to associate an icon with a python class. Someone can explain a bit more how to use it? There is an usege policy? thanks in advance Alberto