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.