diff --git a/doc/system_description/software.md b/doc/system_description/software.md
index be4b301..fd2c45c 100644
--- a/doc/system_description/software.md
+++ b/doc/system_description/software.md
@@ -485,7 +485,7 @@ Assigned core prefixes:
 |                   |       |           |        |          |           | event (reading a 1), write anything here to acknowledge it.            |
 | `FW_RAM`          | r/w   | invisible | 1 kiB  | u8[1024] |           | Firmware-only RAM.                                                     |
 | `UDA`             | r     | invisible | 16B    | u8[16]   |           | Unique Device Authentication key.                                      |
-| `UDI`             | r     | r         | 8B     | u64      |           | Unique Device ID (UDI).                                                |
+| `UDI`             | r     | invisible | 8B     | u64      |           | Unique Device ID (UDI).                                                |
 | `QEMU_DEBUG`      | w     | w         |        | u8       |           | Debug console (only in QEMU)                                           |
 | `NAME0`           | r     | r         | 4B     | char[4]  | "tk1 "    | ID of core/stick                                                       |
 | `NAME1`           | r     | r         | 4B     | char[4]  | "mkdf"    | ID of core/stick                                                       |
diff --git a/doc/system_description/system_description.md b/doc/system_description/system_description.md
index 0358845..052ea3f 100644
--- a/doc/system_description/system_description.md
+++ b/doc/system_description/system_description.md
@@ -65,9 +65,9 @@ The TKey store and use the following assets internally:
   the device. Tillitis will NOT store a copy of the UDS. Can be read
   by firmware once between power cycling
 
-- UDI - Unique Device ID. 64 bits. Provisioned and stored during device
-  manufacturing. Accessible by FW and applications. Never to be replaced
-  or altered during the life time of a given device. May be copied,
+- UDI - Unique Device ID. 64 bits. Provisioned and stored during
+  device manufacturing. Only accessible by FW. Never to be replaced or
+  altered during the life time of a given device. May be copied,
   extracted, read from the device.
 
 - CDI - Compound Device Identity. Dervied by the FW when an application