external-text-entity #syntax OR (when | unless) entity is #syntax
A built-in stream that refers to the concrete syntax in the SGML declaration. Concrete syntax can be either specified in the SGML declaration or provided through a public identifier describing a public concrete syntax.
If a public identifier is provided, the entity text associated with the identifier must conform to the part of the concrete syntax defined by production  in ISO 8879, the SGML standard. This means that the entity text must be what would follow the SGML keyword
syntax in the SGML declaration, but the entity text can't be another public identifier.
#syntax entity must contain something other than white space and comments: all parts of an "external public concrete syntax" must be present.