diff --git a/src/group.h b/src/group.h
--- a/src/group.h
+++ b/src/group.h
@@ -12,7 +12,7 @@
 
 enum {
 	ALL_GROUP     = 0xFFFD,
-	DEFAULT_GROUP = 0xFFFE,
+	DEFAULT_GROUP = 0xFFFE, ///< ungrouped vehicles are in this group.
 	INVALID_GROUP = 0xFFFF,
 };