They can be used under several circumstances. Aerobic is used when oxygen is needed and Anaerobic does not use oxygen. As an example, if you were running on a tredmill at a steady rate, keeping your heart at a steady rate, aerobic respiration would be used. However, if you were playing a sport in which you're going to be stopping and picking up the intensity a lot (i.e. football, basketball, hockey, etc), then anaerobic respiration would be used.