Second part from code review

Signed-off-by: Georg Seifert <georg.seifert@carissma.eu>

Author: DerBaertige

osi_common.proto

@@ -221,20 +221,39 @@ message Identifier
 // This could be other OpenX standards, 3rd-party standards or user-defined objects.
 // simulation environments.
 //
+// \note The ExternalReference is an optional value and can be left empty.
+//
 message ExternalReference
 {
     // The source of the external references
     //
-    // Defines the original source of an object as an URI.
-    // The URI should be follow the syntax according to 
+    // Defines the original source of an object as uniquely identifiable reference.
+    // In case of using \c GroundTruth::map_reference or 
+    // \c GroundTruth::model_reference, the reference can be left empty.
+    // If not otherwise required, an URI is suggested and should follow the syntax according to 
     // \link https://tools.ietf.org/html/rfc3986 RFC 3986\endlink.
     //
+    //
     optional string reference = 1;
 
     // The type of the external references
     //
     // Must be used to describe the type of the original source.
     //
+    // For OpenX/ASAM standards it is specified as follows,
+    // the PATCH-Value is optional:
+    // - OpenDRIVE 1.6
+    // - OpenDRIVE 1.6.0
+    // - OpenDRIVE 1.6.1
+    // - OpenSCENARIO 1.0
+    // - OpenSCENARIO 1.0.0
+    // - OpenSCENARIO 1.1
+    // - OpenSCENARIO 1.1.0
+    //
+    // For third-party standards and user-defined objects the 
+    // Reverse Domain Name Notation is suggested, to guarantee unique
+    // and interoperable identifications.
+    //
     optional string type = 2;    
 
     // The external identifier reference value.
@@ -245,6 +264,19 @@ message ExternalReference
     // E.g. referencing a unique lane in OpenDRIVE 
     // (RoadId --> String, S-Value of LaneSection --> Double, LaneId --> Int)
     // 
+    // \note The detailed description of the identifiers and how they are 
+    //       used for referencing external objects are given in the individual
+    //       messages, where it is deployed.
+    //
+    // \see EnvironmentalConditions::source_reference
+    // \see Lane::source_reference
+    // \see LaneBoundary::source_reference
+    // \see StationaryObject::source_reference
+    // \see MovingObject::source_reference
+    // \see RoadMarking::source_reference
+    // \see TrafficLight::source_reference
+    // \see TrafficSign::source_reference
+    //
     repeated string identifier = 3;
 }
 

osi_environment.proto

@@ -88,7 +88,7 @@ message EnvironmentalConditions
     //
     optional Fog fog = 7;
 
-    // External Reference to the Environmental Condition Sources
+    // Optional external Reference to the Environmental Condition Sources
     //
     // \note For OpenDRIVE and OpenSECNARIO there is no direct counterpart.
     //

osi_lane.proto

@@ -50,7 +50,7 @@ message Lane
     //
     optional Classification classification = 2;
 
-    // External Reference to the Lane Source
+    // Optional external Reference to the Lane Source
     //
     // The ExternalReference point to the source of the lane, if it is derived
     // from one or more objects or external references. An example here is the 
@@ -750,7 +750,7 @@ message LaneBoundary
     //
     optional Classification classification = 3;
 
-    // External Reference to the Lane Boundary Source
+    // Optional external Reference to the Lane Boundary Source
     //
     // \note For OpenDRIVE 1.6 there is no direct possibility to reference the
     //       RoadMark, as there is no unique identifier in this sub-object.

osi_object.proto

@@ -392,7 +392,7 @@ message MovingObject
     //
     optional MovingObjectClassification moving_object_classification = 9;    
 
-    // External Reference to the MovingObject Source
+    // Optional external Reference to the MovingObject Source
     //
     // The ExternalReference point to the source of an moving object, if it
     // is derived from an external sources like OpenSCENARIO.

osi_roadmarking.proto

@@ -55,7 +55,7 @@ message RoadMarking
     //
     optional Classification classification = 3;
 
-    // External Reference to the Roadmarking Source
+    // Optional external Reference to the Roadmarking Source
     //
     // The ExternalReference point to the source of the surface marking, if it
     // is derived from one or more objects or external references. An example

osi_trafficlight.proto

@@ -42,7 +42,7 @@ message TrafficLight
     //
     optional string model_reference = 4;
 
-    // External Reference to the Traffic Light Source
+    // Optional external Reference to the Traffic Light Source
     //
     // The ExternalReference point to the source of the traffic light, if it is
     // derived from one or more objects or external references. An example here

osi_trafficsign.proto

@@ -157,7 +157,7 @@ message TrafficSign
     repeated SupplementarySign supplementary_sign = 3;
 
 
-    // External Reference to the Traffic Sign Source
+    // Optional external Reference to the Traffic Sign Source
     //
     // The ExternalReference point to the source of the traffic sign, if it is
     // derived from one or more objects or external references. An example here