diff --git a/src/postProcessing/functionObjects/field/fieldValues/controlDict b/src/postProcessing/functionObjects/field/fieldValues/controlDict
index 58a7349371..c7619975f7 100644
--- a/src/postProcessing/functionObjects/field/fieldValues/controlDict
+++ b/src/postProcessing/functionObjects/field/fieldValues/controlDict
@@ -68,6 +68,8 @@ functions
         sourceName      movingWall;
 
         //// if sampledSurface: dictionary with a sampledSurface
+        //// Note: the sampledSurfaces will have cell-values, i.e.
+        //// non-interpolated. Also will not sample surface fields.
         //sampledSurfaceDict
         //{
         //    type            cuttingPlane;
diff --git a/src/postProcessing/functionObjects/field/fieldValues/faceSource/faceSource.H b/src/postProcessing/functionObjects/field/fieldValues/faceSource/faceSource.H
index 9d10cdcfce..bae5a21ca6 100644
--- a/src/postProcessing/functionObjects/field/fieldValues/faceSource/faceSource.H
+++ b/src/postProcessing/functionObjects/field/fieldValues/faceSource/faceSource.H
@@ -69,6 +69,11 @@ Description
     faces.
     - all fields get oriented according to the faceZone (so you might e.g. see
     negative pressure)
+    - using sampledSurfaces:
+            - they do not do surface fields
+            - they use cell values - they do not do any interpolation.
+            - take care when using isoSurfaces - these might have duplicate
+              triangles so integration might be wrong
 
 SourceFiles
     faceSource.C