What is a Solid File?

I'm posting this in AI because it seems like the most likely program that would utilize this, based on extensive research. Beyond that, I have no idea what it is or what kind of program it would be used in.

Can anyone tell me what a Solid File is? I've scoured the internet and can't find ANY reference as to what a Solid File is. I know what a Solid Layer is in After Effects, but a Solid File seems to be a ghost term that nobody talks about.

I'm attempting to open a merch store through Streamlabs and they have this ridiculous FAQ that no other merch store, that I can find, makes note of. So I can't tell if it's some sort of Streamlabs quirk or something else. I know how to use layer masks and feathering and whatnot, but I need to know what a Solid File is and how it's put together. Can anybody help explain what it is? Because I can't find jack squat about it. 

Real-time edit: On a whim, I tried a different search and found something about a SOLID file format on file.org. Apparently it is some sort of file format, but that's all it states. I have no idea which programs can create one and it doesn't give any functional information about what a SOLID file actually is or how to create one. It's extremely baffling that there is virtually no information on a Solid File, when apparently they're rather important to tshirt/merch printing.