diff --git a/icu4c/source/i18n/messageformat2_evaluation.h b/icu4c/source/i18n/messageformat2_evaluation.h
index 82655cf7149b..bed6c197dca5 100644
--- a/icu4c/source/i18n/messageformat2_evaluation.h
+++ b/icu4c/source/i18n/messageformat2_evaluation.h
@@ -32,6 +32,28 @@ namespace message2 {
 
     using namespace data_model;
 
+    // InternalValue
+    // Encodes an "either a fallback string or a FormattedPlaceholder"
+    class InternalValue : public UObject {
+        public:
+        bool isFallback() const { return !fallbackString.isEmpty(); }
+        InternalValue() : fallbackString("") {}
+        // Fallback constructor
+        explicit InternalValue(UnicodeString fb) : fallbackString(fb) {}
+        // Regular value constructor
+        explicit InternalValue(FormattedPlaceholder&& f)
+            : fallbackString(""), val(std::move(f)) {}
+        FormattedPlaceholder value() { return std::move(val); }
+        UnicodeString asFallback() const { return fallbackString; }
+        virtual ~InternalValue();
+        InternalValue& operator=(InternalValue&&);
+        InternalValue(InternalValue&&);
+        private:
+        UnicodeString fallbackString; // Non-empty if fallback
+        // Otherwise, assumed to be a FormattedPlaceholder
+        FormattedPlaceholder val;
+    }; // class InternalValue
+
     // PrioritizedVariant
 
     // For how this class is used, see the references to (integer, variant) tuples
diff --git a/icu4c/source/i18n/unicode/messageformat2.h b/icu4c/source/i18n/unicode/messageformat2.h
index c875f762d2ed..7a306d8b96b8 100644
--- a/icu4c/source/i18n/unicode/messageformat2.h
+++ b/icu4c/source/i18n/unicode/messageformat2.h
@@ -32,30 +32,7 @@ namespace message2 {
     class MessageContext;
     class ResolvedSelector;
     class StaticErrors;
-
-    // Internal use only
-    // None = null operand
-    // String = fallback value
-    // FormattedPlaceholder = non-error value
-    class InternalValue : public UObject {
-        public:
-        bool isFallback() const { return !fallbackString.isEmpty(); }
-        InternalValue() : fallbackString("") {}
-        // Fallback constructor
-        explicit InternalValue(UnicodeString fb) : fallbackString(fb) {}
-        // Regular value constructor
-        explicit InternalValue(FormattedPlaceholder&& f)
-            : fallbackString(""), val(std::move(f)) {}
-        FormattedPlaceholder value() { return std::move(val); }
-        UnicodeString asFallback() const { return fallbackString; }
-        virtual ~InternalValue();
-        InternalValue& operator=(InternalValue&&);
-        InternalValue(InternalValue&&);
-        private:
-        UnicodeString fallbackString; // Non-empty if fallback
-        // Otherwise, assumed to be a FormattedPlaceholder
-        FormattedPlaceholder val;
-    }; // class InternalValue
+    class InternalValue;
 
     /**
      * <p>MessageFormatter is a Technical Preview API implementing MessageFormat 2.0.
diff --git a/icu4c/source/i18n/unicode/messageformat2_formattable.h b/icu4c/source/i18n/unicode/messageformat2_formattable.h
index ed396dd87fa6..ab748d7c61d8 100644
--- a/icu4c/source/i18n/unicode/messageformat2_formattable.h
+++ b/icu4c/source/i18n/unicode/messageformat2_formattable.h
@@ -642,7 +642,7 @@ class U_I18N_API ResolvedFunctionOption : public UObject {
          * @internal ICU 75 technology preview
          * @deprecated This API is for technology preview only.
          */
-        FormattedPlaceholder(const Formattable& input, const UnicodeString& fb, UErrorCode& status);
+        FormattedPlaceholder(const Formattable& input, const UnicodeString& fb, UErrorCode& errorCode);
         /**
          * Default constructor. Leaves the FormattedPlaceholder in a
          * valid but undefined state.
@@ -651,7 +651,15 @@ class U_I18N_API ResolvedFunctionOption : public UObject {
          * @deprecated This API is for technology preview only.
          */
         FormattedPlaceholder();
-        // TODO
+        /**
+         * Returns true iff this FormattedPlaceholder represents a null operand
+         * (the absence of an operand).
+         *
+         * @return A boolean indicating whether this is a null operand.
+         *
+         * @internal ICU 75 technology preview
+         * @deprecated This API is for technology preview only.
+         */
         UBool isNullOperand() const { return type == kNull; }
         /**
          * Returns a pointer to