The Global Tax Mod or GTM is a feature in Superpower 2. It basically serves as a way to increase all sectortaxes simultaneously with the given percentage.
Due to the way the GTM works, it's possible to have a maximum of 200% on resources. Any resource with 100% sectortax, coupled with a GTM of 100% would result in a lot of income, with relatively minor consequences to your economy. This would, ofcourse, mean that all resources will have a minimum tax of 100%.
A lot of hosts online demand that players limit their GTM to a certain percentage, or forbid the use of the GTM completely.
Ruges has come up with a way to keep both the GTM convenience, while keeping the maximum tax at 100%. Using the SDK, several mods have limited sectortaxes at a number, and limited the GTM at (100-max sectortax). MultiMOD (all versions until 1.4), for example, limits the GTM at 30%, while limiting sectortaxes at 30%.