This rule enables you to locate the last remaining submarine. If the last submarine is known to reside in both a specific row and a specific column, the submarine is located at the intersection of the row and column.
Example
The board shown above has four submarines to be found.
Three finalized submarines are shown: (G,8), (G,10) and (H,1).
Examining the board shows that the last submarine must reside somewhere along row J. This is because wherever the remaining ship segment in row J is, it is a submarine.
Using similar logic, however, we know the last submarine must also reside along column 10. Again, this is because wherever the remaining ship segment in column 10 is, it is a submarine.
The only location for the last submarine that will satisfy both assertions is the intersection of row J and column 10: square (J,10).