//Translation unit #1:

export module stuff;
export template<typename T, typename U> void foo(T, U u) { auto v = u; }
export template<typename T, typename U> void bar(T, U u) { auto v = *u; }  

//Translation unit #2:

export module M1;
import "defn.h";        // provides struct X {};
import stuff;
export template<typename T> void f(T t) {
  X x;
  foo(t, x);
}

// Translation unit #3:

export module M2;
import "decl.h";        // provides struct X; (not a definition)
import stuff;
export template<typename T> void g(T t) {
  X *x;
  bar(t, x);
}

// Translation unit #4:

import M1;
import M2;
void test() {
  f(0);
  g(0);
}

For f(0), the comment says that

the instantiation context of foo<int, X> comprises

• the point at the end of translation unit #1,
• the point at the end of translation unit #2, and
• the point of the call to f(0),

For g(0), the comment says that

the instantiation context of bar<int, X> comprises

• the point at the end of translation unit #1,
• the point at the end of translation unit #3, and
• the point of the call to g(0),

For every first bullet, the point is specified by [module.context#3]

During the implicit instantiation of a template whose point of instantiation is specified as that of an enclosing specialization ([temp.point]), the instantiation context is the union of the instantiation context of the enclosing specialization and, if the template is defined in a module interface unit of a module M and the point of instantiation is not in a module interface unit of M, the point at the end of the declaration-seq of the primary module interface unit of M (prior to the private-module-fragment, if any).

However, it is confusing about the second bullet for either f(0) or g(0). Which rule specifies that the end of TU2 is comprised by foo<int, X>, the end of TU3 is comprised by bar<int, X>? Specifically, is comprised by the enclosing specialization?

I think the emphasized wording just work for the template whose point of instantiation is specified as that of an enclosing specialization. In other words, [module.context] p3 should be parsed as that

During the implicit instantiation of a template T whose point of instantiation is specified as that of an enclosing specialization ([temp.point]), the instantiation context is the union of the instantiation context of the enclosing specialization and, if the template T is defined in a module interface unit of a module M and the point of instantiation is not in a module interface unit of M, the point at the end of the declaration-seq of the primary module interface unit of M (prior to the private-module-fragment, if any).

Except that the emphasized wording intends to work for the arbitrary template it is saying, it does not make sense the end of TU2 or TU3 is comprised.