Addendum: For a uniqueness pattern to be a valid it must occur over exactly two rows, two columns, and two blocks. Blocks are also known as boxes. For example, this is not a valid uniqueness pattern because it occurs over four separate blocks:
https://imgur.com/kHugeHX
The reason having four blocks does not create a uniquenss problem is if our four cells occur in four separate blocks, the additional shared house within the four blocks will always have values eliminating any ambiguity with uniqueness.
List of all dxSudokus: https://www.youtube.com/channel/UC2N60jDPloV5_imTYKIVH2Q/videos?view_as=public
This video is on the advanced puzzle solving technique call Unique Rectangle Type 3.
For an additional reference on Unique Rectangles:, Hodoku Wiki:
http://hodoku.sourceforge.net/en/tech_ur.php
And this site has a very nice Wiki page on Unique Rectangles:
https://www.sudokuwiki.org/Unique_Rectangles
