diff --git a/gnss/1.0/IGnssNavigationMessageCallback.hal b/gnss/1.0/IGnssNavigationMessageCallback.hal
index 3fdae9f..24ee708 100644
--- a/gnss/1.0/IGnssNavigationMessageCallback.hal
+++ b/gnss/1.0/IGnssNavigationMessageCallback.hal
@@ -119,7 +119,8 @@
          *
          * - For Galileo F/NAV, this refers to the page type in the range 1-6
          *
-         * - For Galileo I/NAV, this refers to the word type in the range 1-10+
+         * - For Galileo I/NAV, this refers to the word type in the range 0-10+
+         * A value of 0 is only allowed if the Satellite is transmiting a Spare Word.
          */
         int16_t submessageId;