It's not stuff in a cargo bay that opens and closes. It's the Cargo category. Usually for when you need to do the (broken) repair this part on that craft missions that you would need to use them. EVA Repair Kits, EVA Science Kit, Small lights, some small solar panels, the drogue parachute that does not need an attachment point (the small one). These are all things that can stack in the Cargo Containers.. or they should, but I can't for some reason. I grab a second one and click on the spot where the first one is and it just clicks. No extra part added, but the Container shows that it would have room for it (in Liters). I was able to do it for a while after it was introduced, but something changed and I'm not sure what. I thought it was just a glitch that would be fixed, but still there after a few months of waiting, so posting here.