Understanding Region of Interest selection in a Camera
The image is scanned (and data stored) with an imaginary cursor starting at the upper left, working across horizontally to the right, then the cursor pauses to move down one line and resetting to left edge. One could think of the rectangular image as a single string where the "carriage return - line feed" is merely implied by keeping track of the timing (or counting pixels). In most image storage formats, the width and height are tags in the header and the data itself is a long stream without demarcated frame extents. Since the figures speak about clocks and blanking however, file storage is probably not implicated here.

I think that the horizontal scan lines can be skipped (or at least started and then reset arbitrarily), but cannot be started at an arbitrary distance from the left edge.

Selection 1, is optimal for this behavior: you send a minimal set of commands to skip down to the proper horizontal position, and scanning can start at the upper left of the desired viewport scan what is desired and then skip the rest.

In Selection 2, we must scan every single line to at least the rightward extent of the part we are interested in, at which time we can send a command to move to the next line and reset.

Very roughly: if you have a 99w x 99h "units", and are interested in 1/3 of the image centered as in the examples, in selection 1, you scan 99w x 33h units while in selection 2, you must scan 66w x 99h units minimum. It is unclear to me, but the supporting figure looks to me to be speaking about selection 2 only, and the horizontal "2,592clk 1H" implies to me that the right-side mask is skipped for scanning.

Note that if the camera cannot start a horizontal line without scanning the complete line (i.e. can't stop and reset at an arbitrary distance from the left edge), then the selection 2 scenario must scan 99w x 99h units.