the 1 in the middle right box must be in the yellow square.
Here is a different example.
The 6 in the top row must be in the yellow square.
Terminology A forced digit is often referred to as a 'hidden single'. (This 'naked' vs. 'hidden' stuff refers to the situation where one has written down all possibilities for all fields. Now if there is only one possibility somewhere, that is immediately apparent, while it requires a scan over a row or column or box to recognize that some digit can only be in one place. But in practice it is too time-consuming to write down all possibilities, and forced digits - hidden singles - are found more easily than forced positions - naked singles.)
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