Rephrase

index.bs

@@ -9687,9 +9687,10 @@ from other interfaces. Specifically:
     will correspond to properties on the object itself rather than on
     [=interface prototype objects=].
 
-A [=realm=] can be defined to
-<dfn export for="realm" lt="has a mutable global prototype chain">have a mutable global prototype chain</dfn>.
-If a [=realm=] is not defined as such, the prototype of its global object can not be changed.
+All [=realms=] have a <dfn export for="realm">is global prototype chain mutable</dfn> flag,
+which can be set when the [=realm=] is created.
+The value of the flag can not change during the lifetime of the [=realm=].
+By default the flag is set to false.
 
 <div class="note">
 
@@ -11502,7 +11503,7 @@ with the [{{LegacyNoInterfaceObject}}] [=extended attribute=].
     1.  Otherwise, set |proto| to |realm|.\[[Intrinsics]].[[{{%Object.prototype%}}]].
     1.  Assert: <a abstract-op>Type</a>(|proto|) is Object.
     1.  Let |interfaceProtoObj| be null.
-    1.  If |realm| [=realm/has a mutable global prototype chain=], then:
+    1.  If |realm|'s [=is global prototype chain mutable=] is true, then:
         1.  Set |interfaceProtoObj| to [$OrdinaryObjectCreate$](|proto|).
     1.  Otherwise, if |interface| is declared with the [{{Global}}] [=extended attribute=], or
         |interface| is in the set of [=inherited interfaces=] of an interface
@@ -11682,7 +11683,7 @@ is the concatenation of the [=interface=]'s
     When the \[[SetPrototypeOf]] internal method of a [=named properties object=] |O| is called with
     ECMAScript language value |V|, the following step is taken:
 
-    1.  If |O|'s [=associated realm=] [=realm/has a mutable global prototype chain=],
+    1.  If |O|'s [=associated realm=]'s [=is global prototype chain mutable=] is true,
         return [=?=] [$OrdinarySetPrototypeOf$](|O|, |V|).
     1.  Return [=?=] <a abstract-op>SetImmutablePrototype</a>(|O|, |V|).
 
@@ -13502,7 +13503,7 @@ Issue: Define those properties imperatively instead.
     [=interface=] with the [{{Global}}] [=extended attribute=] is called with
     ECMAScript language value |V|, the following step is taken:
 
-    1.  If |O|'s [=associated realm=] [=realm/has a mutable global prototype chain=],
+    1.  If |O|'s [=associated realm=]'s [=is global prototype chain mutable=] is true,
         return [=?=] [$OrdinarySetPrototypeOf$](|O|, |V|).
     1.  Return [=?=] <a abstract-op>SetImmutablePrototype</a>(|O|, |V|).