I have no idea what the point of random order is, but sequential (I believe) just means they are in the sequential order of the real thing. They are not limited by difficulty level of questions unless you check one of those boxes.
I always chose sequential, and that format was structured just like the real MCAT was. It alternates between passages and discretes with the final couple of questions being discretes (again, just like the real thing).
I guess I don't know what RANDOM would accomplish, but sequential is NOT progressively more difficult.