diff --git a/aviary/docs/user_guide/mass.ipynb b/aviary/docs/user_guide/mass.ipynb
index 912484e2e..07230f4b6 100644
--- a/aviary/docs/user_guide/mass.ipynb
+++ b/aviary/docs/user_guide/mass.ipynb
@@ -44,7 +44,10 @@
     "Which variables are used depends on which mass estimation subsystem you're using, such as `aircraft:crew_and_payload:mass_per_passenger`, `aircraft:engine:additional_mass_fraction`, etc.\n",
     "\n",
     "Aviary allows for extensive customization and extensions within the mass subsystem.\n",
-    "Users can develop alternative components and integrate them as subsystems with the existing platform."
+    "Users can develop alternative components and integrate them as subsystems with the existing platform.\n",
+    "\n",
+    "## External Mass\n",
+    "Specifying the `external` mass method disables Aviary's core mass group. This allows for external subsystems to completely replace these calculations."
    ]
   },
   {