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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Those are the 9 sub-squares, the one you picked isn't one of them.
Please follow the link to Wikipedia to see an example of sudoku board and what a valid "sub grid" is. Not every 3x3 square of the board is a subgrid where the rules apply, there's only 9 of them.