RealItem + comment by Mikhail to explain native vs regular data

GUI/Model/Device/RealItem.h

@@ -140,6 +140,17 @@ private:
     QString m_presentationType;
 
     // DataItem can be `IntensityDataItem` and `SpecularDataItem` (default `IntensityDataItem`).
+    /*
+      "Native_data" is introduced and used in specular mode to keep the original axis
+      of the dataset. It also stores a copy of original intensity values inside datafield,
+      but these values are always the same for "data" and "native_data".
+
+      At the moment of loading file to RealItem "data" and "native_data" are completely equal.
+      But after linking RealItem to instrument and changing the current units the axis of "data"
+      is changed. When we unlink RealItem from instrument, we restore the original axis from
+      "native_data", because now there is no other way to transform axis without ICoordSystem
+      coming from instrument.
+    */
     std::unique_ptr<DataItem> m_dataItem;
     std::unique_ptr<DataItem> m_nativeDataItem;
     std::unique_ptr<AbstractDataLoader> m_dataLoader;