Any URN that starts with "urn:publicid:..." is a public identifier.

Its a formal way of naming a resource (e.g. a schema).

The specification at the OASIS site will tell you more.

The URN gets mapped to a public identifer string:

• URN Value: urn:publicid:IDN+cde.berkeley.edu:milowski:schemas:example-form:event:200402

• Public Identifier: IDN cde.berkeley.edu//milowski//schemas//example-form//event//200402

So you just use the 'public' element in the catalog.