Sigmod, also known as Rafmod, is a massive collection of mods that introduce a ton of features / fixes into MvM. It was originally created by Sigsegv, and has since been maintained by assorted modders since. The version used on SCG is maintained by Rafredek. The first Proof of concept version was posted on January 4th, 2016, and continues to get regular updates to this day.
The description originally given for this mod is, "Gigantic, obese SourceMod extension library of sigsegv's TF2 mods (mostly MvM related)". It is built as a framework to interact with elements of TF2 / MvM such as bot pathing, AI, objective handling, popfile handling, and so on. Later versions introduce things like custom attributes for weapons, changes to overall gameplay, balance, bug fixes, and more. Overall, it is used to create missions that can run custom text, vehicles such as blimps, custom weapons, objectives, pathing changes, map manipulation, gameplay changes, class limits, command settings, and so on.
See missions that use Sigmod