diff --git a/Development.md b/Development.md
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@@ -1,3 +1,7 @@
 # Resetting devices
 
-Tuhi stores the device data in `$HOME/.local/share/tuhi/<btaddress>` (i.e. `$XDG_DATA_HOME`) The primary file is `settings.ini` which is how Tuhi remembers devices it has already seen. Removing that device and/or the whole folder makes the device unknown again and the device has to be re-registered.
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+Tuhi stores the device data in `$HOME/.local/share/tuhi/<btaddress>` (i.e. `$XDG_DATA_HOME`) The primary file is `settings.ini` which is how Tuhi remembers devices it has already seen. Removing that device and/or the whole folder makes the device unknown again and the device has to be re-registered.
+
+# BlueZ ManufacturerData
+
+There's an oddity about the `ManufacturerData` property on the devices. That property is a 7-byte value when the device is running normally but a 4-byte value when the device is in register mode. The update of that property is delayed though, so a device may be in register mode before the `ManufacturerData` reverts to 4 bytes. And, crucially, it **may stay in 4-byte mode** for a successful registration but switches to 7 bytes on the next connection. When a registration times out, the device may stay in 4-byte or 7-byte `ManufacturerData`, it depends on some previous state.
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