I was trying to solve the problem supposing that I have to choose a square sub-board of the given chess board from either top-left side or bottom-right side.

But in the problem it is given:

A sub-board of Chef’s chessboard is a rectangular piece of this board with an arbitrarily chosen top left and bottom right cell (possibly equal to the original chessboard). Every sub-board is also a chessboard.

So what exactly is said in the problem, by “arbitrarily chosen top left and bottom right cell”, can anyone explain it with some example.

Thanks