filter results by region/domain?

(from github.com/amahajan87)
Is it possible to have a custom filter in addition to different kinds of documents?

(from github.com/marevol)
Yes.
To add a custom field to a reponse, see https://github.com/codelibs/fess/issues/1634#issuecomment-386439257.
If you want to use it as a search field, you can add it to query.additional.search.fields.