Someone will give a better answer but some vehicles will use the post catalytic sensor occasionally to adjust fuel trims when it sees readings that suggest that the cat can't handle the amount of fuel the normal trims is applying.
Getting involved in discussions because I have a lot to learn still.
My general rule is, if the vehicle is equipped with an air/fuel ratio sensor, is has downstream fuel control. If it has an garden variety upstream O2 sensor, it may or may not have downstream fuel control.
How much control will depend on the specific year/make/model. For example, many Ford models I see have downstream fuel control, but it's very limited in its amount of control. 2% +/-, usually.
Upstream sensors and catalysts degrade over time with use. The downstream sensor is used (among other things) to adjust for the sensor and catalysts changes over time. For some years now, OBD2 systems are required to monitor the ability of the downstream sensor to perform this function.
It is notable that "fuel control" as described by just about everyone isn't true and hasn't been for more than 25 years. 30+ years ago there were numerous white papers and patents discussing the problems with "reactive" fuel control and its inability to fully utilize the catalyst.
We teach and use what we do because these (trim etc) are the tools given to us. I teach that it is critical to deal with trim numbers under steady state conditions (at the minimum)
Generic OBD gives us ST and LT trim control values. Yet, OEs don't necessarily "do" fuel control that way. I've been told that they are not happy with having to generate this "conversion" as it isn't necessarily correct. But for the most part it is accurate enough if we stay away from transitional values and deal with DTCs instead of trying to outsmart the onboard system.
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