A re-breathing mask has a soft plastic reservoir bag attached at the end that saves one third of a person’s exhaled air, while the rest of the air gets out via side ports covered with one-way valves.
This allows the person to “re-breathe” some of the carbon dioxide which acts as a way to stimulate breathing.
Non re-breathing mask
A non-re-breathing mask has several one way valves in the side ports. This type of mask also has a reservoir bag attached, but the bag has a one way valve that prevents the exhaled carbon dioxide from getting into the reservoir.
This type of mask does not allow for the re-breathing of exhaled air as it escapes through the side ports.
The purpose of such a mask is to deliver high amounts of oxygen in an emergency eg hypoxia, respiratory disease, cardiac disease etc.