I believe this referred to the value of an automatically-calculated, since-removed column on the Devices page of the hub (similar to that of the "classic" SmartThings IDE), the "Status" column. This happened a long time ago, and I think it took a long time after that for anyone to even notice. 
Here's an old screenshot from the old docs, no longer available, this being column "E":
It was removed, I believe, precisely because there was no good way for the hub to determine this value (I think it was just an hours-based calculation, the same for any device and thus rarely accurate for all). The corresponding property on the device object apparently still remains, but I'd be surprised if it wasn't read-only. In any case, I don't know of anywhere that it is used, and I further wouldn't be surprised to see it marked as deprecated some day. Likely just a leftover from initial efforts to make ST ports easier...
