This app is new variant of Sudoku *1. This variant is called "Tatami Sudoku". Tatami Sudoku has some small blocks. These are spread like Japanese tatami mat. Therefore, we call them "Tatami block".
"Tatami block" has created a very difficult tactical solving in Tatami Sudoku. Maybe Tatami Sudoku is more difficult than standard Sudoku.
Block of "SDK Tatami" is different from Standard Sudoku. Please see screenshots. 7x7 sized board has 9 blocks. 8x8 has 11 blocks, and 9x9 has 13 blocks. Digits cannot repeat on each column, row and block.