Map extent represents the bounding box of a map when the rotation angle is set to 0 degrees.

Rotated map extent represents the rectangular bounding box of a map containing the rotated map extent. At 0 degrees the rotated map extent is equal to mapextent. As the rotation angle changes the rotated extent is scaled to fully contain the map extent as well as areas outside map extent visible within the map control. At 45, 135, 225, and 315 degrees the rotated extent reaches its largest size. The rectangular bounding box of the rotated extent also covers areas that are not visible within the map control.

ViewExtent is a polygon that represents the bounds of the map control. It covers the portion of the rotated map extent that is visible within the map view.

The illustration below shows difference between extents at 0 and 45 degree angles. At 0 degrees all extents are coincidental. At 45 degrees map extent and rotated extent cover areas both within the map view as well as outside of it.

Reference