In Application management> Portals> Portals you can set filter fields under the Search settings tab: 'General filter fields' and 'Document filter fields'.
In addition to documents, you can also link report forms, reports, hyperlinks, questionnaires and cards from a card catalog to your portal via a collection.
All these 'items' have a title, a type and are included in a collection.
Those three aspects are the "general filter fields".
For documents and processes, you can create extra fields that are only filled in for documents. These are the "Document filter fields".