diff --git a/src/Storages/MergeTree/MergeTreeSequentialSource.cpp b/src/Storages/MergeTree/MergeTreeSequentialSource.cpp
index 5075e43448a..ba50447be0f 100644
--- a/src/Storages/MergeTree/MergeTreeSequentialSource.cpp
+++ b/src/Storages/MergeTree/MergeTreeSequentialSource.cpp
@@ -20,7 +20,9 @@ namespace ErrorCodes
 }
 
 
-/// Lightweight (in terms of logic) stream for reading single part from MergeTree
+/// Lightweight (in terms of logic) stream for reading single part from
+/// MergeTree, used for merges and mutations.
+///
 /// NOTE:
 ///  It doesn't filter out rows that are deleted with lightweight deletes.
 ///  Use createMergeTreeSequentialSource filter out those rows.