A unique solution
Every live DIGIGMA puzzle has one solution. There is one way to finish the grid. If a mark is right, it is the only mark that can sit there. That is a fact of the product, not a slogan. The site does not hand you a board that can end two ways.
Uniqueness changes how you sit. You are not exploring options. You are reading a single world that is already decided, poorly lit. The pleasure is not that you invented an ending. It is that the ending was necessary, and you found it.
One finish
A unique-solution puzzle is a closed object. Every empty cell has one value that works with the rules. If two finished grids both looked legal, the puzzle would be broken — not “interesting,” broken. Designers throw those boards away. Players should never have to vote.
On DIGIGMA that contract holds for Sudoku, Killer Sudoku, and Binary Puzzle. The clues (digits, cages, 0s and 1s already placed) are enough. Nothing required is missing. Nothing extra is hiding in the author’s head.
Elimination
Because only one finish is legal, you can erase. If a candidate would leave two ways to complete a group, it is wrong. If a fill would force a clash in a row, a cage, or a line of three 1s, drop it. You do not need a second solution to appear on the page. You need to notice that a candidate would create a fork, or would break a rule. Then that candidate goes.
That is why Draft marks get smaller. You are not choosing between two real worlds. Only one world is on the board. A leftover 7 that cannot sit in this box is not an opinion. It is a cell you can stop watching.
Contradiction
Sometimes the useful sentence is negative. “If this cell were 5, the cage could not sum.” “If this line ended like that, it would copy the row above.” You never have to play the fiction out to the last cell. A short, local contradiction is enough. The unique solution is what makes the fiction illegal instead of merely unlucky.
This is not a technique name to collect. It is the same look you already use on Easy, applied to a slightly longer chain. The how-tos teach the first scans. Uniqueness is why those scans are allowed to conclude.
Difficulty is not ambiguity
A hard puzzle is not a puzzle with two answers. Hard means the chain is longer: this digit lives here, so that one cannot, so the next cell is forced. Easy means the forced cells sit on the surface. Both have one finish.
Ambiguity would be a different product — a prompt, a preference, a vibe. DIGIGMA is not that. If you are stuck, you have not found the next legal step yet. You have not been abandoned in a maze of equally good endings. Leave it, look again, open a lower band. Do not flip a coin and call it style. Why guessing feels worse is the sibling page.
The last cells
You feel uniqueness most at the end. The remaining marks fall out. The rules hold. There is no audience and no other grid hiding in the clues. The board agrees with you, or it does not. That agreement is a calm kind of pleasure. You were not choosing among endings. You were finding the only one.
A fast sit is not worth more than a slow one. The finish is the same object. The clock, if you watch it, is a private measure of the evening, not a verdict on the deduction.
The three boards
Sudoku hides uniqueness in placement: once each in row, column, box. Killer Sudoku adds cages, so uniqueness also lives in which sets of digits can make a total. Binary Puzzle makes uniqueness visible as a law of its own: rows and columns must all differ, on top of halves and no-three-in-a-line.
The sensation changes. The contract does not. If you want the first scans, they are here: Sudoku, Killer Sudoku, Binary Puzzle. If you want the three sits compared, Three kinds of board.