Generic global OBD2 provided this PID so that the short term fuel mixture percentage that is being corrected by a post catalyst mixture sensor would be presented.
The OEM's have much proprietary science invested in fuel control strategy. Particularly the use of the post catalyst sensor for controlling average lambda. Every one I have looked at (hundreds) all read 99.x. This I take to mean that they all received an exemption from CARB so that the data is not required to be displayed. If it were displayed, it would help the competition understand when and how much the rear sensor is used thus revealing much about the strategy.
I see no practical use for this data in everyday testing even if the data was correct. There are at least two other ways to verify sensor operation.