diff --git a/Simulation/DetSimSD/include/DetSimSD/DDG4SensitiveDetector.h b/Simulation/DetSimSD/include/DetSimSD/DDG4SensitiveDetector.h
index b4084be54dc9851cde44dc239f8820ba5660387b..687771e344b9f12ae553e56888b1222288fefbaf 100644
--- a/Simulation/DetSimSD/include/DetSimSD/DDG4SensitiveDetector.h
+++ b/Simulation/DetSimSD/include/DetSimSD/DDG4SensitiveDetector.h
@@ -52,7 +52,9 @@ public:
     //  from the current sensitive volume to the world volume
     virtual long long getCellID(const G4Step* step);
 
-    // Returns the position of center of cell
+    // Returns the position of center of cell, according cell ID
+    // if cellID not set, getCellID(step) will be invoke
+    // unit mm
     virtual dd4hep::Position getNominalPosition(const G4Step* step, const long long cellID=0);
 
 protected: