Fat and carbohydrate make up the majority of fuel used at rest and during exercise. Protein can be a source of energy too, but without sophisticated measurements including nitrogen excretion, we can't really measure it.
But, we can make a safe assumption that under normal conditions protein use is negligible, so we can ignore it anyway.
Here's a term you may already be familiar with: steady state. Remember, that's a state where a person is aerobically producing ATP as quickly as they are using it. During steady state, things like VO2, ventilation, heart rate and blood pressure don't change to any great extent. They remain steady.
During steady state aerobic exercise, ATP is primarily produced by the breakdown of energy nutrients: carbohydrate and fat.
What if we wanted to find out just how much carbohydrate or fat we are using? Do we need some invasive type of equipment or procedure (ie. muscle biopsy) to do it? The good news is we don't, all we need to do is measure VO2 and VCO2.
You may also be familiar with the term respiratory exchange ratio or RER. Aerobic energy comes from the oxidation of carbohydrate and fat. We can indirectly identify which of these two nutrients is being oxidized through the measurement of the RER as follows:
VCO2/VO2
Remember, VO2 is oxygen consumed and VCO2 is carbon dioxide produced; that's just how aerobic metabolism works.
How is this related to carbohydrate or fat? For that we have to review chemistry.