diff --git a/commons/src/main/java/org/archive/io/warc/WARCConstants.java b/commons/src/main/java/org/archive/io/warc/WARCConstants.java
index e88894dc..0efa25b8 100644
--- a/commons/src/main/java/org/archive/io/warc/WARCConstants.java
+++ b/commons/src/main/java/org/archive/io/warc/WARCConstants.java
@@ -20,6 +20,7 @@
 package org.archive.io.warc;
 
 import org.archive.io.ArchiveFileConstants;
+import org.archive.modules.writer.WARCWriterProcessor;
 
 /**
  * WARC Constants used by WARC readers and writers.
@@ -194,7 +195,20 @@ public interface WARCConstants extends ArchiveFileConstants {
     public static final String HEADER_KEY_ETAG = "WARC-Etag";
     public static final String HEADER_KEY_LAST_MODIFIED = "WARC-Last-Modified";
     public static final String HEADER_KEY_REFERS_TO = "WARC-Refers-To";
-    public static final String HEADER_KEY_REFERENCE_LOCATION = "X-WARC-Reference-Location";
+    
+    /**
+     * Information that helps to find the record that
+     * {@value #HEADER_KEY_REFERS_TO} refers to. This header is a heritrix
+     * extension not defined in the WARC spec version 1.0. Current
+     * implementation writes a JSON string that looks something like this:
+     * {"original-url":"http://archive.org/robots.txt"
+     * ,"warc-file-offset":1976,"warc-filename":
+     * "WEB-20120912001108855-00000-6882~desktop-nlevitt.sf.archive.org~6440.warc.gz"
+     * ,"content-written-date":"2012-09-12T00:11:10Z"}.
+     * 
+     * @see WARCWriterProcessor
+     */
+    public static final String HEADER_KEY_REFERENCE_LOCATION = "WARC-Reference-Location";
     
     public static final String PROFILE_REVISIT_IDENTICAL_DIGEST = 
         "http://netpreserve.org/warc/1.0/revisit/identical-payload-digest";
diff --git a/modules/src/main/java/org/archive/modules/writer/WARCWriterProcessor.java b/modules/src/main/java/org/archive/modules/writer/WARCWriterProcessor.java
index 6fbdc71b..4985310a 100644
--- a/modules/src/main/java/org/archive/modules/writer/WARCWriterProcessor.java
+++ b/modules/src/main/java/org/archive/modules/writer/WARCWriterProcessor.java
@@ -692,9 +692,16 @@ public class WARCWriterProcessor extends WriterPoolProcessor implements WARCWrit
         headers.addLabelValue(
                 HEADER_KEY_TRUNCATED, NAMED_FIELD_TRUNCATED_VALUE_LENGTH);
         
+        /*
+         * ISO 28500 WARC ISO standard draft says: "The WARC-Refers-To field may
+         * also be used to associate a record of type 'revisit' or 'conversion'
+         * with the preceding record which helped determine the present record
+         * content."
+         */
         headers.addLabelValue(HEADER_KEY_REFERS_TO, 
                 curi.getContentDigestHistory().get(A_WARC_RECORD_ID).toString());
         
+        // {"original-url":"http://archive.org/robots.txt","warc-file-offset":1976,"warc-filename":"WEB-20120912001108855-00000-6882~desktop-nlevitt.sf.archive.org~6440.warc.gz","content-written-date":"2012-09-12T00:11:10Z"}
         @SuppressWarnings("unchecked")
         JSONObject refLoc = new JSONObject((HashMap<String,Object>) curi.getContentDigestHistory().clone());
         refLoc.remove(A_CONTENT_DIGEST_COUNT);