Lookahead depth
How many guesses ahead the solver searches while trying to minimize
the worst-case number of possibilities left, and the number of
guesses needed to finish. At every level, every one of the
1296 codes is considered as a candidate guess - even codes
already ruled out, since an "impossible" guess can sometimes split
the remaining possibilities better than any guess that could still
be the answer.
This is exhaustive and has no time limit.
Depth 1 is instant. Depth 2 typically takes a few seconds, but can
take roughly a minute on the first couple of guesses (when many
possibilities remain). Depth 3 can take a very long time -
potentially hours early in the game - and the server cannot
respond to anything else while it's thinking. Use 2 or 3 only when
you're prepared to wait (a "Stop waiting" button lets you give up
on the page without affecting the in-progress computation).