Etterna scores every note based on how inaccurate, in milliseconds, each hit is. The Judge Difficulty determines the strictness of this scoring system. Judge Difficulty also changes the hold and tap timing windows for Hold Notes and Roll Notes, but it does not change the timing window to trigger a mine. The Judge Difficulty can be changed in the Player Options menu. The default value is 4, which is referred to as Judge 4 or J4.
The shape of the Score Curve determines how much players are penalized for different amounts of inaccuracy. Etterna currently uses the WIFE3 score curve. This score curve is continuous unlike the score curves used by Stepmania and many other rhythm games, i.e. a Perfect with 44ms of error does not offer significantly more points than a Great with 46ms of error.
Wife 3 Score Curve for Judge 4. Image credit: ulti_fd
WIFE3 awards 100% for any hit that is sufficiently close to 0ms. On judge 4, this window is 5ms. Misses are worth -275%, Dropped Holds and Rolls are worth -225%, and triggered Mines are worth -350%.
Any tap within 75ms of a mine will trigger the mine. However, if a player taps more than 75ms before a mine and holds the keypress, the mine will not trigger until it crosses the receptor. For a quick overview of mine mechanics, refer to this video on mine mechanics (credit: Foxfire)
Each judge difficulty is a scalar multiple of J4:
J7 is a special case that moves each judgment up a grade compared to J4. For example, a J7 perfect is a J4 marvelous and a J7 great is a J4 perfect. A J7 marvelous is also called a Ridiculous.
Some timing windows do not change across Judge Difficulties due to unintended effects on game mechanics. The Bad window is always fixed to 180ms to ensure that cbrush mechanics are consistent across difficulties. The roll window is capped to 250ms at higher difficulties to prevent it from turning into a longjack.
Etterna's Judge Difficulties are almost the same as the ones used in Stepmania with a few key differences. Stepmania does not use a fixed 180ms window for Boos/Bads. Etterna disables judge difficulties 1 through 3 because the looser hold/roll release timings cannot be accurately converted to an equivalent J4 score.