Mod files (mods) are charts that apply effects to the chart to make it more challenging to read. Mod files can make the chart bounce around, move up and down, and more. Simple mod files can be creating using Attacks in the Stepmania 5 editor. Many modern mod files are created using code in the Lua programming language, and can only be played on NotITG.
There are tournaments involving playing mod files, the most well-known is UKSRT (UK Sight Reading Tournament).