Class 2 and 3 circuits are governed by Art. 725 and Chapter 9 Table 11A and 11B show some of the differences between them. Lower voltages and wattage ect.. Typically for controls and such. 450.14 wont apply.
Class 2 is power limited, such that shorting the output terminals won't cause a fire. Class 2 is for doorbells, garage door openers, etc
Class 3 has higher power and voltage limits, and its not a very common system