diff --git a/api-ref/source/v3/ext-backups.inc b/api-ref/source/v3/ext-backups.inc
index 05a8305e159..b1a02572fb2 100644
--- a/api-ref/source/v3/ext-backups.inc
+++ b/api-ref/source/v3/ext-backups.inc
@@ -182,6 +182,10 @@ Restores a Block Storage backup to an existing or new Block Storage volume.
 The name parameter will work only if a new volume is created.
 
 If UUID is specified, the backup will be restored to the specified volume.
+The specified volume has the following requirements:
+
+    * the specified volume status is ``available``.
+    * the size of specified volume must be equal to or greater than the size of backup.
 
 If no existing volume UUID is provided, the backup will be restored to a
 new volume matching the size and name of the originally backed up volume.