Candidate
The set of digits that can potentially go in a cell. Recorded as pencil marks, they form the foundation for all solving techniques.
A candidate is a digit that could potentially be placed in a specific cell. After eliminating digits already present in the cell's row, column, and block, the remaining digits form the candidate set. For intermediate and advanced Sudoku, accurate candidate management is the foundation of every technique.
How to Calculate Candidates
The candidates for cell (r, c) are {1,2,...,9} minus all digits present in row r, column c, and the block containing the cell. Whenever a digit is confirmed, candidates in related cells must be updated.
Importance of Candidate Management
Without accurate candidates, advanced techniques like Naked Pairs and X-Wings cannot be applied. Digital apps calculate them automatically, but when solving on paper, manual management is required. Missed updates are the leading cause of incorrect solutions.