Including my thoughts here for the discussion (Peter and I discussed this earlier):
Manufacturer specific PIDs are supposed to be the same for all devices from the same manufacturer, Thus, as long as you can get the PARAMETER_DESCRIPTION for the Sub-device's PID from somewhere, it will give you the information you need.
Thus, you could send a GET PARAMETER_DESCRIPTION to the root to get the description for a PID that is only supported in the Sub-Device(s).
Just because the root gives you a description for a manufacturer specific PID doesn't automatically mean that the root supports that PID.
While reading through the E1.20 doc, the same question applies to STATUS_ID_DESCRIPTION. It can only be sent to the rood, so you'd have to ask the root for the description of a status code that was returned from a sub-device. This may be less of a concern though, since STATUS_MESSAGES should only be sent to the root device.
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