Difficulty is a measure of how physically challenging a chart is to complete. All charts in ITG are assigned a numeric difficulty rating, sometimes called a block level, to give an estimate of how physically demanding it is to execute a chart well enough to avoid failing.
The original/"stock" ITG difficulty scale ranged from 1 (easiest) to 13 (hardest), with only three Singles charts (The Expert charts for Pandemonium, Summer ~Speedy Mix~, and VerTex^2) and three Doubles charts (The Expert charts for !, Energizer, and VerTex^2) being rated a 13. This scale was largely based on and extrapolated from the original difficulty scale from DanceDanceRevolution (prior to the scale rework that took place in DanceDanceRevolution X) which ranged from 1 to 10, with a "10" in ITG and a "10" in DDR being roughly the same level of challenge.
Simfile authors may assign difficulties for each chart in a file according to their best judgment, with other "established" custom charts or the original ITG officials serving as reference points. As a result, there is inherent subjectivity in the rating process, and most players acknowledge that defining exact thresholds for "borderline" charts can be difficult.
In the modern "post-ITG" era of pad customs, there is no defined "upper limit" for difficulty - At the time of writing, charts rated as high as 32 on Singles[1] and 20 on Doubles[2] have been confirmed to be passed.
There are five "standard" difficulty slots for a given simfile, commonly referred to as Novice, Easy, Medium, Hard, and Expert. In addition to these five, simfiles may have additional charts placed in Edit slots, though this practice is very uncommon in most custom files today. These difficulty slot names, along with the numeric rating of the chart in a given slot are used as part of common shorthand used when discussing stepcharts to ensure clarity.
(Example: A Singles chart in the Expert difficulty slot with a difficulty rating of 11 can be referred to as "SX11." A Doubles chart in the Medium difficulty slot with a rating of 6 can be referred to as "DM6.")
