Unclued Information (extraction)

Unclued Information or Unchecked Information is a type of extraction, usually used on clue-based puzzles like Crosswords.

Puzzle Application
Unclued Information, in contrast to many other extractions, focuses heavily on things that should be present in a puzzle, but are conspicuously absent; it extracts information that either goes completely unclued in a puzzle that clues everything else, or information that doesn't get cross-checked in a puzzle that otherwise cross-checks every other entry. In this way, it relies on solvers noticing changes to a formula that they spend a significant amount of time abiding by during the solving process.

A common way of using this extraction is in crossword puzzles, where it's relatively easy to simply leave out a number when labelling a grid, allowing the placement of other entries to define the letters that go in the unnumbered/unclued space. Similarly, one could choose to hide a message across individual letters in puzzle types like Acrostics, keeping certain cells unclued and relying on solvers filling in the gaps in other ways (such as filling in the rest of the message).

Strategy
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Examples

 * - In this puzzle, the central horizontal entry of each of the two grids is completely unclued, and gets almost completely filled in by the crossing vertical entries. While these entries don't constitute the final answer, they do clue the next step in the puzzle.
 * - A Spiral Grid crossword, this puzzle has entries running both inwards and outwards along a linear path. However, in each direction there are some letters that are only clued going that way, and are skipped when going the opposite direction. These letters can be extracted to spell a final answer.