FORTS is a basebuilding supplement for FIST.
The PDF is 4 pages, densely organized, but very clean and readable.
Contents-wise, think of this like the group playbook from Blades In The Dark. You choose a base type, you invest coin in upgrades, and you get resources and allies and other support out of growing your base.
There's a lot of granularity and detail here, and anyone who's a fan of the last mainline Metal Gear will find something in the mix that they love. There's recruitment, resource management, an expertise system. It's comprehensive without being overwhelming.
Overall, this is a fantastic addition to a FIST campaign. If you want to show the PCs growing and changing not just as characters but as a geopolitical force, I strongly recommend plugging this into your game.