[Zope] - type codes in Searchable Object Interface

The Searchable Object Interface documentation at: http://www.digicool.com/site/PrincipiaDeveloperPack/SearchableObjectInterfac... says that fields are types "s", "n", "i", or "d", but doesn't explain what these are. Looking at the b3 ZSQL source, "s" is string, "d" is date, "n" is "number", and "i" seems unused. I though perhaps "n" was for floats and "i" for integers, but since "i" is unused, should I use "n" for both?