diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index 32b4cc84e79..b153fad594d 100644
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -399,6 +399,8 @@ XLogRegisterBufData(uint8 block_id, char *data, int len)
  * - XLOG_MARK_UNIMPORTANT, to signal that the record is not important for
  *	 durability, which allows to avoid triggering WAL archiving and other
  *	 background activity.
+ * - XLOG_INCLUDE_XID, a message-passing hack between XLogRecordAssemble
+ *	 and XLogResetInsertion.
  */
 void
 XLogSetRecordFlags(uint8 flags)
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 77187c12beb..ee3e369b79f 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -246,7 +246,7 @@ extern bool XLOG_DEBUG;
  */
 #define XLOG_INCLUDE_ORIGIN		0x01	/* include the replication origin */
 #define XLOG_MARK_UNIMPORTANT	0x02	/* record not important for durability */
-#define XLOG_INCLUDE_XID		0x04	/* include XID of top-level xact */
+#define XLOG_INCLUDE_XID		0x04	/* WAL-internal message-passing hack */
 
 
 /* Checkpoint statistics */
diff --git a/src/include/access/xlogrecord.h b/src/include/access/xlogrecord.h
index 80c92a2498a..f68cb189128 100644
--- a/src/include/access/xlogrecord.h
+++ b/src/include/access/xlogrecord.h
@@ -211,8 +211,9 @@ typedef struct XLogRecordDataHeaderLong
  * Block IDs used to distinguish different kinds of record fragments. Block
  * references are numbered from 0 to XLR_MAX_BLOCK_ID. A rmgr is free to use
  * any ID number in that range (although you should stick to small numbers,
- * because the WAL machinery is optimized for that case). A couple of ID
- * numbers are reserved to denote the "main" data portion of the record.
+ * because the WAL machinery is optimized for that case). A few ID
+ * numbers are reserved to denote the "main" data portion of the record,
+ * as well as replication-supporting transaction metadata.
  *
  * The maximum is currently set at 32, quite arbitrarily. Most records only
  * need a handful of block references, but there are a few exceptions that