syntax Using
  | UsingOpen  = "using" NamespaceOrType ";";
  | UsingAlias = "using" Name "=" NamespaceOrType ";";

Nitra Using directives are very similar to the ones used in C#. Their purpose is to shorten names.
UsingOpen lets you open up a Namespace, a SyntaxModule and a type, after which the opened character can be addressed with a short name. For example:

using Nitra;
NSpan(1, 2)

UsingAlias you to create an alias for a type or SyntaxModule. For example:

using Token = Nitra.NSpan;
Token(1, 2)