What is a Solid File?

[Rebel Squadron]

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.

[Doug A Roberts · Accepted solution]

They are using 'solid' there as the opposite of transparent, not as a description of a file type.

[Rebel Squadron]

Thank you for that information.

Can you explain why using a layer mask with feathering would be any different than using a transparent background?

[Doug A Roberts]

It may be to do with alpha channels vs. transparency. But I don't think it's possible to know what they mean exactly based only on the information you posted.

[Rebel Squadron]

FINALLY got a coherent response from Streamlabs. According to them, it has to do with the transparent background somehow causing the actual graphic image to appear as partially transparent. 

Here's what I'm thinking, though. I'm thinking that they've had issues with people using the Paint Bucket feature. Using the bucket to dump transparency onto an image background, which then causes the main image to become partially transparent. I had this issue early on in my graphic artist career (such as it is), which forced me to find better ways to create transparent backgrounds. This is all supposition, but personal experience would explain it in this way. Thank you for the information and swift response. You've been immensely helpful!