The "M" at the end of LFXS28566M stands for "matte," as in "matte black stainless steel." It's a new finish from LG, and it falls right in line with the current trend of dark-tinted metallic appliances.
You also get a pantry drawer at the bottom of the fridge with three temperature settings. They didn't seem to make much difference in our tests, though -- drawers like these perform a lot better when they're separated out from the main body of the fridge.
The marquee feature in the body of the fridge has to be the Door-in-Door compartment. Push the button on the right handle, and you'll open the front panel of the right door, exposing the in-door shelves without actually opening the door itself.
You can also sync the fridge with the Google Assistant to check the fridge status or change its settings using voice commands. The syntax is pretty clunky, though. You have to say "talk to LG" before giving your command.
These are the average temperatures in each section of the fridge over a 72-hour test at its default setting of 37 degrees. The main body and left door did fine -- but that Door-in-Door compartment is way too warm.
We ran the coldest setting test a second time, but instead of opening and closing the doors for 5 minutes twice a day to simulate usage, we opened and closed the Door-in-Door compartment for 5 minutes twice a day. Very little changed, which suggests that the Door-in-Door compartment doesn't make much of a positive impact on performance.