The phase of puzzle solving where a player is looking for a solution after having understood all the rules.
Work refers to the phase of puzzle solving in which they know all the pieces involved and have understood the rules of the game. It's the part of a puzzle where a player is applying the rules of the game trying to work out a solution.
For example, a player has recognized they have a paint-by-numbers puzzle and they have all the colours and rules by which to paint. They have to actually paint the image, and this part is called work.
If there is too much repetitive work the player may consider it unappealing, labelling it as grunt work. This happens if there is no inherent fun in the work, or it is too much in contrast to the other phases of the puzzles.
This can be very genre dependent, and often a defining aspect of the genre. Mono-puzzles, such as sudoku, often focus entirely on the work aspect of the puzzle. Adventure games and escape rooms tend to have little work, favouring more the discovery aspects of puzzles.
Whether work is appropriate also depends on the player's abilities verse the challenge of the work. For example, an experienced sudoku player would not enjoy beginner puzzles, as would a beginner merely become frustrated at advanced puzzles.
When looking at a puzzle from the view of the phases, puzzles tend to either be work focused, or light on work.
A player may attempt to do the work for a puzzle without having all the rules they need. Or they may have misunderstood some aspect of the puzzle pieces. In a work-light puzzle the player will quickly discover they are missing something. In a work-heavy puzzle the player may waste a lot of time, uncertain of whether they made a mistake in the work, or are missing information.
Assume everything in this reference is a working draft, there's prone to be some mistakes and inconsistencies. I figure it's best to publish and get feedback rather than write for years in secret. The terms will change, the structure will shift, and the bugs will be chased out. It'll take a while.