diff --git a/db/dbformat.h b/db/dbformat.h
index 1c86b127a55461af2293438741f37e3c7fffc59e..1647661b8bbd47333b9c021afe7a1f7e339fa11f 100644
--- a/db/dbformat.h
+++ b/db/dbformat.h
@@ -32,6 +32,8 @@ enum ValueType : unsigned char {
   kTypeDeletion = 0x0,
   kTypeValue = 0x1,
   kTypeMerge = 0x2,
+  // Following types are used only in write ahead logs. They are not used in
+  // memtables or sst files:
   kTypeLogData = 0x3,
   kTypeColumnFamilyDeletion = 0x4,
   kTypeColumnFamilyValue = 0x5,