It's a container and what's inside can be almost anything. This is why you should never see anyone claim to support all MKV and AVI playback. The MKV and AVI is supported but not what's inside the container.
This will continue to confuse many for years to come but your supplier of the content will have to tell you if it's ready to play on this set.
I've been downloading and watching 1080p movies in .mkv format by USB/USB HDD and they all have worked fine. Today I downloaded a movie in .mkv but when I try and play it in my TV it says "Video codec not supported".