Utilities: add note on naming ambiguity

Core/include/Acts/Utilities/ParameterDefinitions.hpp

@@ -23,6 +23,13 @@
 #else
 namespace Acts {
 
+// Note:
+// The named indices are use to access raw data vectors and matrices at the
+// lowest level. Since the interpretation of some of the components, e.g. local
+// position and the inverse-momentum-like component, depend on additional
+// information the names have some ambiguity. This can only be resolved at a
+// higher logical level and no attempt is made to resolve it here.
+
 /// Components of a bound track parameters vector.
 ///
 /// To be used to access components by named indices instead of just numbers.