Last Remaining Cell
Last Remaining Cell is the positional single: in one row, column, or box, a particular digit has only one possible cell. It is the same deduction commonly called a Hidden Single. The cell may still display other candidates.
What is Last Remaining Cell?
Choose one digit in one row, column, or box and consider all of its remaining positions. If every position but one has been ruled out by placed digits or earlier sound deductions, the digit must occupy the remaining cell. This differs from Last Free Cell, which requires a sector with eight placed digits.
Finding the cell
Start by scanning the board for a digit that's already been placed a lot — something like a 2 or 5 that shows up in several boxes. For each house where it's still missing, mentally block off the rows, columns, and boxes that already contain it. Whatever cell is left at the intersection is your candidate. This pairs naturally with Last Free Cell: placing a digit in one house often creates a Last Remaining Cell in a neighbor. Solvers use it to crack open sections that look stuck.
Combine with scanning
The proof uses a digit-to-position view: every sector must contain the digit exactly once, and only one position remains. Written candidates are a convenient record rather than a prerequisite. The same candidate may also be a Naked Single if its cell has no other candidates.