Note, this whole post assumes that we are only talking about front shifters/derailleurs.

I use 2 of the Fd-R443. They are for mountain bike shifters, NOT road shifters/brifters unless the road shifter/brifter is designed to pull more cable than a standard road shifter.

For the most part (i'm sure there is an exception), a road index shifter (brifter, downtube, bar end, whatever) pulls quite a bit less cable per front shift point than a respective mountain bike front shfiter.

You would use a FD-R443 when you have a road style front crankset (50t or larger) but you want to use a mountain bike shifter. The problem with the great majority of mountain bike derailleurs is that the cages are designed for 48t size chain rings at the max. You try using a mountain bike front derailleur with a 52t front crankset and you run into shifting problems especially with triples.

Hence the need for the non-series FD-R443 style derailleurs. The cages are designed for large 50t and larger chainrings but will index properly with the larger cable pulls from flat bar/mountain bike shifters.

You would not use a FD-r443 when using a road bike style shifter such as a brifter.

I have 2 bikes that were converted FROM drops to flat bar and have road sized chain rings and flatbar shfiters and used the FD-R443.