Good question, but I’m afraid I don’t have an answer. I’m in the UK and we’ve never had NC since it was launched.
That is the rain from yesterday (midnight to midnight in station time zone) with no NC corrections. You will probably find a lot of the answers to your original question here: Where can I find out what these "obs" values mean?.
I suppose the point here is that if the documentation makes no reference to a particular field/value then it is undocumented. I’m sure you have seen this, but the documented parts of the API are described here: Tempest API. Since joining WF way back when it was originally launched, I can think of only one change that was made to the documented API: the addition of the NC fields to the obs array for the observation end point