The language allows implementations to define other instantiable types. For example, a foreign language interface might want to import a type from another language as something other than a Dylan class, but still allow it to be used as the first argument to make.

Can users, as well as implementations "define other instantiable types"?  Or, rather, is there something here that implementations can do but users can't?

Users can create new types (instances of <type>) that are by default uninstantiable (singletons, limited types, union types) and can then define make methods for them, making them instantiable, although I'm not sure that should fall into recommended practice.

Users can't create new kinds of types;  that is, users cannot create new subtypes of <type> in the portable language.

Well, I suppose I can define a new instantiable type like so:

  define $days = #[
         #"sunday", #"monday", #"tuesday",
         #"wednesday", #"thursday", #"friday",
         #"saturday" ];

  define constant <day> :: <type> =
         reduce1(union, map(singleton, $days));

  define method make
     (type == <day>, #key ordinal, #all-keys)         $days[ordinal]
  end method make;

but I'm not recommending that.

> Or, rather, is there something here that implementations can do
> but users can't?

Yes. Making new direct subclasses of <type>. For example, Dylan defines union types, but not intersection types. There is no portable way I know of to portably create an intersection type which can be used as a method specializer.