The Matroska specs have been modified to add a new value to DisplayUnit. So far noone used the field. But it was either interpreted as pixels (correct) or display aspect ratio (incorrect). So the new value for Display Aspect Ratio (DAR) has been added to clarify the situation. It should not have any impact on any existing player. But if you have a muxer, make sure you are writing pixels or a DAR with the right DisplayUnit value.