Step 1: Recall how subsidence grows with panel width.
As an underground panel gets wider relative to its depth, the amount the surface sinks directly above its centre keeps increasing, but only up to a certain point.
Step 2: Identify the turning point.
At the critical width, the centre point of the surface just reaches the maximum subsidence value possible for that seam thickness, this is the threshold where the point subsidence first hits its ceiling.
Step 3: See what happens beyond that width.
If we keep extracting wider than this critical width, the subsidence value at the centre cannot rise any further since it is already at maximum, but now this maximum value spreads sideways and forms a flat bottomed trough covering an entire area rather than just a single point.
Step 4: Match this to the terminology.
This condition of the maximum subsidence being sustained over an area, not just a point, is precisely what is called super critical width of extraction.
\[ \boxed{\text{Super Critical width of extraction}} \]