I've noticed in multiple tags there is a function section. and some shaders and light volumes can be controled by functions. Here are a few of my questions

1. what are they used for
2. how do they work

I believe functions are simple flags/variables to make things dynamic. For instance, the light on the front of the SP banshee. It is controlled by a function which checks whether there is a driver or not. On the heavy scorpion, the chainguns on the bottom are controlled by a function which checks for integrated light power toggle, or the flashlight button.

However, I only understand these for vehicles. I know there are functions for weapons too, and those may be for things like holding down triggers and switching (like in the plasma pistol) and age, but I am not sure.

There are 8 functions able to be set. The first 4 are input functions, which is things like driver power and flashlight. Then the output functions are the ones in the "functions" sections. These control things like attachments, and can have input functions as parameters.

Someone will have to verify this, but I also think there are global functions, like your health or shields. You can make the tag modify these somehow, I think.