Rob, the Primos "Can" call is a doe in estrus call. It mimics the sound of a doe in heat (estrus) and should be used only during the rutting period. It says it can be used during all phases of the rut.
If used during the wrong time of the year a buck knows something isnot right, and willbe easily spooked. Do you find this not accurate?
The differant sizes are for more volume.