What is the difference between different coil duty cycles?

The continuous duty ratings means that the coil can remain energized indefinitely without damage to the coil. Intermittent duty coils have more power which improves switching characteristics, but because more power is generated than the coil can dissipate, they cannot be energized indefinitely. There are different intermittent duty ratings for the various series:

J46 Series

Prolonged: Up to 90% duty cycle; 50 minutes maximum on time; a little more force than continuous duty Intermittent: 50% duty cycle; 15 minutes maximum on time; more force than continuous and prolonged duty High Power: Up to 25% duty cycle; 3 minutes maximum on time; the highest force available.

Plastic Case & Metal Case series

Heavy Duty Intermittent: 25%; 10 minutes maximum on time Intermittent: 25% duty cycle; 2 minutes maximum on time; more force than HD intermittent