diff --git a/receiver/JFJochReceiverPlots.h b/receiver/JFJochReceiverPlots.h
index 9985a10f..f1171c5d 100644
--- a/receiver/JFJochReceiverPlots.h
+++ b/receiver/JFJochReceiverPlots.h
@@ -62,6 +62,10 @@ class JFJochReceiverPlots {
     StatusVector max_value;
     StatusVector resolution_estimate;
 
+    // StatusVector objects are fully thread-safe (protected by internal mutex)
+    // It is OK to have concurrent access to StatusVector
+    // roi_m lock is needed to make sure that std::map<std::string, ROIStatus> is not mutable within critical section
+    // so no new elements added outside of a unique lock, but it is OK to modify ROIStatus under shared lock
     struct ROIStatus {
         StatusVector sum;
         StatusVector max_count;