diff --git a/src/ansys/api/acp/v0/material.proto b/src/ansys/api/acp/v0/material.proto
index 029f852..defb777 100644
--- a/src/ansys/api/acp/v0/material.proto
+++ b/src/ansys/api/acp/v0/material.proto
@@ -219,6 +219,9 @@ message Properties {
         FabricFiberAnglePropertySet fabric_fiber_angle = 17;
     }
     PropertySets property_sets = 4;
+
+    // Transfer ID (read only)
+    string ext_id = 5;
 }
 
 message ObjectInfo {