I'm not too knowledgeable about this terminology specifically, but I'll pitch in. Generally a 'sandwich' refers to two pieces of bread + whatever you can stuff in between. The difference between a burger and a sandwich is that the burger contains a hot element. Feel free to correct me if anyone knows for sure.

Americans do use 'sandwich' to refer to a burger. Like a sandwich - a burger is after all a filling between two bits of bread. But we here in New Zealand, Australia, and I believe the UK would look at you strangely if you used 'sandwich' to refer to a burger. and BTW - three bits of bread and two fillings is called a 'club sandwich'.

You can also use the term sandwich figuratively meaning to squeeze something in to a very small space of available time frame. Eg. It was so crowded on the train - I was sandwiched between two big men and could hardly move! I'll see if I can sandwich it in between the morning meeting and having to leave for the airport at 1.00pm . Also used to indicate that you are caught between two warring parties - My parents were always arguing about whether I should have to learn the piano, and I was the meat in the sandwich.

Thank you Haley!

It's just referring to the burger.