diff --git a/MdePkg/Include/Protocol/AtaPassThru.h b/MdePkg/Include/Protocol/AtaPassThru.h
index 33d0e21b84..29987a2321 100644
--- a/MdePkg/Include/Protocol/AtaPassThru.h
+++ b/MdePkg/Include/Protocol/AtaPassThru.h
@@ -270,6 +270,10 @@ EFI_STATUS
   If PortMultiplierPort is the port multiplier port number of the last ATA device on the port of
   the ATA controller, then EFI_NOT_FOUND is returned.
 
+  When port multiplier is not connected to the Port, GetNextDevice() may either return
+  EFI_SUCCESS and set PortMultiplierPort to 0xFFFF or return EFI_NOT_FOUND (in which case the
+  PortMultiplierPort value is undefined).
+
   @param[in]     This                A pointer to the EFI_ATA_PASS_THRU_PROTOCOL instance.
   @param[in]     Port                The port number present on the ATA controller.
   @param[in,out] PortMultiplierPort  On input, a pointer to the port multiplier port number of an