diff --git a/js/common/model/Mass.ts b/js/common/model/Mass.ts
index 887e937e..a8a11295 100644
--- a/js/common/model/Mass.ts
+++ b/js/common/model/Mass.ts
@@ -638,6 +638,8 @@ export default abstract class Mass extends PhetioObject {
     },
     applyState( mass: Mass, obj: MassIOStateObject ) {
 
+      // Some of the following attributes are not public, but are settable since this IO Type is declared as a static
+      // class member. This is preferable to making the attributes public everywhere.
       const SIZE_SCALE = DensityBuoyancyCommonQueryParameters.p2SizeScale;
 
       mass.matrix.set( Matrix3.fromStateObject( obj.matrix ) );