diff --git a/packages/MtpDocumentsProvider/src/com/android/mtp/MtpDatabase.java b/packages/MtpDocumentsProvider/src/com/android/mtp/MtpDatabase.java
index df9d44ac37554..df4a9f0b5325e 100644
--- a/packages/MtpDocumentsProvider/src/com/android/mtp/MtpDatabase.java
+++ b/packages/MtpDocumentsProvider/src/com/android/mtp/MtpDatabase.java
@@ -20,8 +20,15 @@ import java.nio.charset.StandardCharsets;
  * Database for MTP objects.
  * The object handle which is identifier for object in MTP protocol is not stable over sessions.
  * When we resume the process, we need to remap our document ID with MTP's object handle.
- * The database object remembers the map of document ID and fullpath, and helps to remap object
- * handle and document ID by comparing fullpath.
+ *
+ * If the remote MTP device is backed by typical file system, the file name
+ * is unique among files in a directory. However, MTP protocol itself does
+ * not guarantee the uniqueness of name so we cannot use fullpath as ID.
+ *
+ * Instead of fullpath, we use artificial ID generated by MtpDatabase itself. The database object
+ * remembers the map of document ID and object handle, and remaps new object handle with document ID
+ * by comparing the directory structure and object name.
+ *
  * TODO: Remove @VisibleForTesting annotation when we start to use this class.
  */
 @VisibleForTesting