There is now a clear difference between Xiaomi product lines as Xiaomi CEO lei jun explained vividly on Weibo the difference between the MI and Redmi. Xiaomi has decided to discontinue Mi Max and Mi Note series as stated by the CEO that the MI max and MI note wouldn’t have successors this year.

Lei Jun said MI brand would continue to focus on high-performance models with the latest advents like Mi9 and the upcoming CC range. One Popular thing the Mi Max series is known for; is its large display and rugged battery while the Mi Note series hold the stand of Xiaomi upper mid-range line.
From the recent Redmi K20 released some months back, the K20 is the successor to the MI Note except for the large display, and hopefully, the Mi Max series might succeed their newer brand like CC range.
However, there has not been an official update from Xiaomi to discontinue the Mi Max and Mi Note series because Lei Jun specified that the MI brand would continue to focus on high-performance feature. And so there might be a plan to expand the brand, so we have to sit back and watch.