diff --git a/samples/bluetooth/direct_test_mode/README.rst b/samples/bluetooth/direct_test_mode/README.rst
index ade8737047a5..fe6a5e07afba 100644
--- a/samples/bluetooth/direct_test_mode/README.rst
+++ b/samples/bluetooth/direct_test_mode/README.rst
@@ -8,6 +8,8 @@ Bluetooth: Direct Test Mode
    :depth: 2
 
 This sample enables the Direct Test Mode functions described in `Bluetooth® Core Specification <Bluetooth Core Specification_>`_ (Vol. 6, Part F).
+Also, the actual encoding of the test commands and events are described in Vol. 6, Part F, section 3.3 of this specification document.
+The `Vendor-specific packet payload`_ section describes some vendor-specific commands, which comply with the core specification.
 
 Requirements
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